Design of optimized mobility capabilities in future 5G systems

Design of optimized mobilitycapabilities in future 5G systems

Aalborg University

Master Thesis, September 2014 - June 1015Group 1053 - Wireless Communication Systems

Institute of Electronic Systems

Department of Electronic Systems

Niels Jernes Vej 12 A5

9220 Aalborg Øst

Phone 9635 8650

http://es.aau.dk

Title:

Design of optimized mobility capabilitiesin future 5G systems

Project period:Master Thesis, 1 September 2014 - 3 June 2015

Group:1053

Members:

Maria Carmela Cascino

Maria Stefan

Supervisors:Andrea CattoniKlaus PedersenLucas Chavarria Gimenez

Number of pages: 69

Number of appendix pages: 15

Number of annex pages: 1 (CD)

Handed In: 3/6 2015

Abstract:

The focus for this thesis is improving the mobilityperformance in high speed and achieving high datarate. The study departs from analysing the existingLTE cellular system both at low and high speedsby means of real field measurements. Simulationscenarios have been created for the LTE system inorder to check if the generated results are reliablecompared to the real measurements. From these itis found that LTE cannot support a thousandfoldincrease in data rate. Thus, an additional 5Gnetwork layer to the existing LTE aims to supportthis data rate increase.

In this project the 5G network deployment is based

on high frequency, ultra dense small cells and high

bandwidth. The mobility study is based on dual con-

nectivity between different radio access technologies:

LTE and 5G. Results from simulations show that the

time-to-trigger has a major impact in the mobility

performance and that high connectivity to the 5G

small cells improves the throughput experienced by

the users in the scenario.

Report content is freely available, but publication (with references) may only be done after agreement with the

authors.

Preface

This report written by group 1053 who are 10th semester students at the department of WirelessCommunication under the institute of Electronics Systems, Aalborg University. The theme forthis project is “ Design of optimized mobility capabilities in future 5G systems”. The projecthas been devised in the period September, 1. 2014 to June, 3. 2015.

Reading Instructions

This report is addressed to supervisors, students and others who are interested in the field ofWireless Communication. By extension the report presumes the reader to have a basic knowl-edge within this field.

The report contains references following the Harvard-method; [Surname,Year]. These referencespoint to a bibliography in chapter 8 (p. 70). The bibliography contains information about thesource like author, title, year of release etc.

Figures and tables are numbered according to their location in the report. Lists of figures andtables can be found in chapter 8 (p. 70). Unless otherwise specified, the figures and tables inthe report have been produced by the project group. References to these will be followed by apage number unless they are on the current page.

Formulas and calculations will also be numbered according to their location. The presentationof these are inspired by the ISO-31 standard for technical communication which, amongst otherthings, means that the SI-unit system will be used.

Appendices and Annex CDThroughout the report, references to annex and appendix will occur. All material contained inthe appendix is made by the project group unless otherwise specified. The annex CD containsother material relevant for the report. This includes relevant literature such as articles andnotes. An overview of the annex can be found in Chapter 9 and the annex itself is placed onthe CD attached to the report. The appendix is placed in the back of the report. References tothese will occur when they are relevant.

II

Contents

1 Introduction 1

2 LTE Overview 42.1 LTE System Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2 Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3 Carrier Aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3 LTE Performance at Low Speed 233.1 Measurement analysis Aalborg city center . . . . . . . . . . . . . . . . . . . . . . 233.2 Measurements campaign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.3 Simulation versus real measurements . . . . . . . . . . . . . . . . . . . . . . . . . 303.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4 On the way towards 5G 354.1 Measurement analysis Aalborg highway . . . . . . . . . . . . . . . . . . . . . . . 354.2 Heterogeneous Networks approach . . . . . . . . . . . . . . . . . . . . . . . . . . 464.3 5G overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5 5G Implementation 505.1 Network Presentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505.2 Path loss models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515.3 Mobility with Carrier Aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6 Optimizing the scenario 616.1 Radio Link Failure evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616.2 Frequency evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 636.3 Small cell events optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

7 Conclusion and Future Work 68

8 List of references 70

9 Annex Index 75

A Phone messages 77A.1 Measurement report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77A.2 RRC Connection Reconfiguration . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

B Overview of OFDMA and SC-FDMA 81B.1 OFDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81B.2 SC-FDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

C Statistics of the configured events for Aalbog city center scenario 86C.1 Statistic for Cells Operating in 1.8 Ghz . . . . . . . . . . . . . . . . . . . . . . . 86C.2 Statistic for Cells Operating in 2.6 Ghz . . . . . . . . . . . . . . . . . . . . . . . 90

Abbreviations and Nomenclature

BS Base Station

CDF Cumulative Distribution Function

DL Downlink

EPC Evolved Packet Core Network

E-UTRAN Evolved Universal Terrestrial Radio Access Network

CDF Cumulative Distribution Function

FTP File Transfer Protocol

GSM Global System for Mobile Communications

NetNet Heterogeneous Network

HO Handover

HOF Handover Failure

HSPA High-Speed Packet Access

ISD Inter Site Distance

LOS Line Of Sight

LTE Long Term Evolution

MIMO Multiple Input Multiple Output

MME Mobility Management Entity

NLOS Non-Line-Of-Sight

P-GW Packet Data Network Gateway

RACH Random Access Procedure

RAT Radio Access Technology

RLF Radio Link Faileure

RRC Radio Resource Control

RSRP Reference Signal Recieved Power

RSRQ Reference Signal Recieved Quality

RSSI Received Signal Strength Indication

S-GW Serving Gateway

SINR Singnal to Interference Plus Noise Ratio

TTT Time to trigger

UDN Ultra Dense Network

UE User Equipment

WCDMA Wideband Code Division Multiple Access

IV

Chapter 1

Introduction

In the recent years, the mobile technology evolution has been achieved thanks to different fac-

tors: for instance, the cost of the devices has become accessible to everyone compared to the

past. The remarkably increasing number of mobile subscribers in the recent years has developed

the need for ubiquitous coverage and high capacity. Nowadays terminals are not only used for

phone calls, but they can also be connected to internet and provide a wide range of services. For

example, with the introduction of streaming services like Netflix or Video-on-Demand (VOD)

the demand of traffic is growing exponentially as well as the media quality. If the traffic growth

continues, the actual capacity will not be able to support the demand [Holma and Toskala, 2009].

For this reason, a major effort has been spent in the recent years in the development of next-

generation wireless communication systems that will bring higher data rate and system capacity.

One of the crucial challenges nowadays is to ensure a reliable connectivity and, at the same

time, a high data rate when a user is travelling at high speed. When the User Equipment (UE)

is moving towards one cell, before the radio link signal of the actual serving cell beomes weak,

it is necessary to move the connection to the cell from which the UE receives a better signal

level. This operation is called handover and it becomes extremely difficult when the UE speed

increases. In fact, if the handover fails, the UE loses the connection and experiences a so called

connection dropping. Therefore, choosing the right condition that triggers the handover event

is essential. All these factors determine the performance of the mobility.

The researchers started to investigate a new technology beyond the actual Long-Term Evolution

(LTE). In fact, projects like ”Mobile and wireless communications Enablers for the Twenty-

twenty Information Society” (METIS) have the objective of laying the foundation for the next-

generation technology named 5G. Even though there is not a clear idea of what the 5G will be,

the main assumptions are related with the very high carrier frequency, a large bandwidth, ultra

dense network (small cells) and massive MIMO [Jeffrey G. Andrews et al., 2014]. With these

configurations a higher data rate can be reached but, at the same time, the mobility has to be

redefined especially when high speed is involved. The 5G technology aims to reach at least 10

Mbps in any kind of environment [Ericsson, 2015].

For these reasons, with this work is wanted to investigate the mobility performance in a high

speed scenario where small cells are involved, as well as the throughput experienced by the UE.

Several field tests in the existing LTE cellular system are performed. Simulations are created

for these tests based on operator and measurement information. The reason for performing

1

simulations is to evaluate if the model used to generate the results is reliable. Furthermore,

these simulations will be used as a baseline for introducing 5G elements in the scenario. The

steps for the study are as follows:

� LTE field measurements in the city center of Aalborg at low speed (15 km/h) for evaluating

the LTE performance

� simulation of the LTE system in low speed used to validate the implemented model

� LTE field measurements on the highway in Aalborg at high speed (100 km/h) also for LTE

performance evaluation

� simulation of the LTE system in high speed in order to verify if the LTE system can support

an increase in data demand. It is expected that the data rate should increase 1000 times

more for the traffic demand [Jeffrey G. Andrews et al., 2014].

� addition of a 5G network to the existing high speed scenario with the aim of achieving

high throughput for the UE. The additional network consists of densely deployed small

cells along the path followed in the field measurements.

The 4G network layout for the highway is used as a layer of macro cells while along the highway

small cells are deployed, constituting a HetNet. A new feature from LTE is used, called carrier

aggregation, that combines different carrier frequencies in order to increase the bandwidth and

therefore the data rate.

The presented work has been carried out focusing on the theoretical background of LTE, on its

real performance in the real network and on the implementation and testing of the proposed

HetNet scenario where 5G is also involved.

Together with high data rate another aim of this study is the optimization of the mobility per-

formance in the scenario that contains 5G. For this study, concepts like Ultra Dense Networks

(UDN), increased frequency and dual connectivity will be implemented and their impact on the

developed system will be investigated. High data rate is difficult to achieve in high mobility in a

very dense heterogeneous network. Therefore, it is assumed that deploying small cells operating

at high frequency and high bandwidth play an important part in achieving high data rate at

the user end. Given the deployment of the scenario consisting of two layers operating at differ-

ent carrier frequencies that belong to different Radio Access Technologies (RAT), the mobility

performance will be analysed. Furthermore, methods to improve this mobility performance and

enhance the dual connectivity between base stations belonging to different RATs will be studied.

The mobility performance radio parameters such as Reference Signal Received Power (RSRP),

Reference Signal Received Quality (RSRQ), Received Signal Strength Indication (RSSI), through-

put are some of the Key Performance Indicators (KPI) that will be used to evaluate the perfor-

mance of both the existing LTE system and the newly implemented scenario. Other KPIs that

are discussed relate to radio link failures, handover failures and ping-pong effect.

2

The delimitations for the scenario consisting on LTE and 5G are as follows:

� use of carrier aggregation and HetNet

� having a minimum of 10 Mbps throughput satisfaction for a high percentage of the users

in the system

� constant user speed of 120 km/h

� carrier frequency of the 5G layer of cells in the range of 6-20 GHz

� inter site distance for the small cells of 100 meters

� overall bandwidth for the small cells of 100 MHz for 10 Physical Resource Blocks (PRB)

The parameters for creating the network of small cells are chosen according to the 3GPP stan-

dard ([TR36.814, ]) and METIS tests ([METIS et al., 2015]).

The thesis is organized as follows: Chapter 2 gives an overview of the LTE in terms of system

architecture, mobility and carrier aggregation. Chapter 3 illustrates the results from the mea-

surements performed in Aalborg city center on 4G network as well as the comparisons with the

simulations. Chapter 4 highlights the performance from the measurements done in the highway

on the area of Aalborg and presents the basis and motivation for the new technology (5G).

Chapter 5 explains the developed scenario in terms of propagation models and mobility. It also

describes the simulation results and the performance. Chapter 6 presents the enhancements

brought to the previous implementation by redefining some of the parameters. Their impact on

the scenario is analysed and the obtained results can give an insight of what parameters and

network deployment can be utilized while advancing towards 5G.

3

Chapter 2

LTE Overview

In order to have a good overview of the mobile system, this chapter explains the history evolu-

tion of the mobile communications from the first generation (1G) to the latest version of LTE

(Long Term Evolution), the latest version of mobile telecommunication systems. The purpose of

this introduction is to give an overview of the LTE system and its advantages compared to the

previous technologies. The following topics will include the architecture of LTE, the mobility

functionality in LTE and methods that can enhance user association.

The first mobile communication system arrived in 1980 named ”First Generation” (1G) which

was using an analogue technology with bulky equipment and comprise a number of indepen-

dently developed systems worldwide.

The ”Second Generation” (2G) also known as GSM (Global System for Mobile communications),

arrived with the introduction of digital system communication. GSM was mainly designed for

voice traffic while data appeared later with the introduction of GPRS/EDGE. All these tech-

nologies were based on Time- and Frequency- Division Multiple Access (TDMA/FDMA).

Later the data usage became more remarkable, thus a ”Third Generation” (3G) was developed

in order to satisfy the market demand. This was possible thanks to the introduction of CDMA

(Code Division Multiple Access) in U.S. and WCDMA (Wideband Code Division Multiple Ac-

cess) in Europe.

In December 1998, the Third Generation Partnership Project (3GPP) was created; 3GPP

was formed by standards-developing organizations from all regions of the world that had the

aim of ”co-operate in the production of globally applicable Technical Specification and Technical

Report for a 3rd Generation Mobile System based on evolved GSM core network and the radio

access technology that they support” [3GPP Agreement 4 December 1998, www.3gpp.org].

In the following years various upgrades have been made to 3G, such as High Speed Down-

link Packet Access (HSDPA) in 2002, High Speed Uplink Packet Access (HSUPA) in 2004 and

HSPA+ in 2007. During 2009, 3GPP initiated the work on 3GPP Long-Term Evolution (4G),

aiming to take a step forward in terms of system performance and service capabilities.

In the Figure 2.1 (p. 5) it is shown the evolution of the mobile communications standards land-

scape from the GSM technology up to latest version of LTE.

4

CHAPTER 2. LTE OVERVIEW

Figure 2.1: Timeline of the mobile communications standards landscape[Stefania Sesia and Baker, 2011]

The next in the 3GPP standardization is LTE Release 8, providing improved data performance

compared to the previous systems. Afterwards, Release 9 introduces new elements in the archi-

tecture. This is followed by LTE-Advanced, specified in 3GPP Release 10, which allows the use

of carrier aggregation in heterogeneous networks and aims to improve the performance of the

users at the cell edge.

With the introduction of every new technology it has been also possible to gradually increase

the data traffic. The evolution of the data rate is shown in Figure 2.2.

Figure 2.2: Evolution peak data rate [Holma and Toskala, 2009]

LTE-Advanced is designed to improve the capabilities of LTE in terms of data rate, capacity

and coverage. Thus, this technology will be used for the purpose of this project. Further on,

the following chapter will give an insight in the LTE architecture, mobility procedure and the

carrier aggregation technique, since these will be the main topics covered in the following study.

5


2.1 LTE System Architecture

This section will give an insight in the architecture and protocols of the LTE System, underlying

the basic functions and utilities of the system components.

LTE was designed to provide seamless connectivity between the User Equipment (UE) and the

Packet Data Network (PDN), without the UE experiencing any disruption in its applications.

The user is able to access the Internet at the same time is makes a voice call for example (VoIP),

while the network ensures the user keeps it security and privacy.

The basic architecture of the LTE Release 8 can be depicted in Figure 2.3. It consists of four

main levels: User Equipment (UE), Evolved Universal Terrestrial Radio Access Network (E-

UTRAN), Evolved Packet Core Network (EPC) and the Services domain. The first three layers

in the architecture scheme, UE, E-UTRAN and EPC, represent the Evolved Packet System

(EPS) whose main function is to provide IP connectivity [Holma and Toskala, 2009].

Figure 2.3: Basic System Architecture Overview [Holma and Toskala, 2009]

The development in the E-UTRAN LTE architecture is related to the evolved NodeB (eNodeB),

also known as the base station. E-UTRAN is a mesh of eNodeBs connected to each other

through the X2 interface. All radio related protocols are terminated in the eNodeB. Its role is to

avoid sending the same data IP header repeatedly. It needs to be connected to the neighboring

eNodeBs to which the handover may be made. As it can be seen in Figure 2.3, the eNodeB acts

as a bridge between the UE and the EPC, connecting the eNodeBs to other logical functions of

the EPC. The eNodeB relays the data between the radio connectivity and the IP connectivity

6


towards the EPC. It also monitorizes the resource usage and makes handover decisions for the

UE. This connection can also be depicted in figure 2.3 (p. 6) [Holma and Toskala, 2009].

Within the EPC, MME, S-GW and P-GW can be found:

� The Mobility Management Entity (MME) in EPC keeps track of the users location

in the Tracking Area and is the main control function of EPC. It also controls the signal-

ing in the handoveror cell change between eNodeBs, S-GW and MMEs, when the UE is

in connected mode. If the UE is idle, then it will report its location periodically or when

it moves to another Tracking Area [Holma and Toskala, 2009]. The protocols running be-

tween the UE and EPC are known as Non Access Stratum (NAS), which establishes the

connection and security between the UE and the network [Stefania Sesia and Baker, 2011].

� Another component of the EPC is the Packet Data Network Gateway (P-GW). It

allocated IP adresses to the UE when it requests a Packet Data Network (PDN) connec-

tion. Therefore, the P-GW acts as a router between the EPS and external packet data

networks. P-GW is the highest level mobility anchor in the system. When a UE moves

from one S-GW to another, the bearers have to be switched in the P-GW. The P-GW will

receive an indication to switch the flows from the new S-GW [Holma and Toskala, 2009].

� The Serving Gateway (S-GW) is used to transfer the IP packets when the UE moves

between eNodeBs. Its main role is to control the user plane tunnels by routing and for-

warding user data packets. It receives commands from the MME to switch tunnels between

eNodeBs [Stefania Sesia and Baker, 2011].

� Communication is protected by eavesdropping using the authentication function in the

Home Subscriber Server (HSS). This function assures that the user is who it claims

to be. It also stores the identities of the used P-GWs

The purpose of this section was to give an overview of the main LTE architecture components

and their main functionalities, as they play an important role in the mobility process, which will

be described in the next section.

7


2.2 Mobility

This chapter gives an insight in the LTEmobility procedure, which is useful for the purpose of this

project and will make the subject for measurements that will be presented in the next chapter.

Describing the mobility in LTE is essential for understanding the procedure and parameters used

to make it work, being able afterwards to analyse the outcome of the performed measurements.

2.2.1 Mobility Overview

To get information about the available channels in the selected area, the UE sends a random

request over the medium, also known as random access procedure (RACH) which will be detailed

further in section 2.2.5 (p. 15).

There are two mobility procedures in LTE: RRC idle and RRC connected

[Stefania Sesia and Baker, 2011]. RRC stands for Radio Resource Control protocol, which is in

charge of the signalling between the user and the eNodeB. First of all the idle mode procedure

will be described, then the connected mode and how the handover is performed. The main focus

will be on the connected mode procedure, since this is the main topic for this thesis.

� Idle mode Mobility Procedure

In idle mode, the mobility is controlled by the UE, which decides and performs cell se-

lection and reselection based on its measurements. First, the UE selects a suitable cell for

camping from a selected network, according to radio measurements.

After the cell selection, the UE will keep checking if there are better cells. This process is

known as reselection. During the idle periods, the UE measures the signal quality of the

neighbours it can receive, then it decides the best connection based on different parame-

ters(e.g. signal quality, priorities). If the UE does not manage to find a suitable cell for

selection, it will look for a cell in another network [Holma and Toskala, 2009].

In LTE, priority is a new parameter. The eNodeB can provide a priority per LTE fre-

quency and per RAT. In case the UE camps on a cell with the highest priority frequency,

the UE doesn’t need to measure other cells as long as the signal strength is above a certain

threshold. Otherwise, if it camps on a low cell priority, it should do regular measurements

for other cells with high priority frequency/RAT [Holma and Toskala, 2009].

In 3GPP Release 9, the cell reselection rule is based on cell absolute priority. This means

that the cells operating on the same carrier frequency have equal priority and the cell

selection is based on the signal level and quality. In case the reselection process is between

different radio access technologies (RAT) or inter-frequency reselection (layers), each layer

is assigned a different priority. The reselection is then made towards the highest priority

8


RAT/frequency that can provide good service to the UE [Salo, 2013].

� Connected mode Mobility Procedure

The transition to RRC connected mode is done as long as there is an active data session

or call setup. In cellular communication systems, a cell has limited coverage area. When

a UE is connected to a cell, it is possible to move out of a cell’s coverage while being in

a call session. The process by which the session is transferred to another cell is called

handover. This is done in order to avoid the interruption when the UE gets outside the

coverage of the first cell.

The handovers are performed while the UE is in connected mode and they are based on

radio measurements. The handovers are controlled by the network, which decides to which

cell a UE should connect in order to maintain the radio link quality. In the early releases

of LTE, the UE can connect to one cell, meaning that before connecting to the second

cell, it breaks the connection to the first one. This is the concept of hard handover. With

the Release 10 of LTE, it is possible to establish dual connectivity with cells operating on

different frequencies. This concept is known as carrier aggregation. Hard handover still

happens in Release 10 due to the use of OFDM. The carrier aggregation technique will be

the subject for the next section [Holma and Toskala, 2012].

During this switching of the UE from one eNodeB to another, some packet transmission issues

may occur. In the downlink direction, the retransmission of unnecessary packets by the target

eNodeB happens if the source does not acknowledge their reception at the UE. It is the UE

which identifies and removes the duplicate packets. It may also happen that the same packets

are sent twice in the uplink direction, this problem is then being solved in the packet core

[Holma and Toskala, 2009].

2.2.2 Measurements in LTE

The topic for this section is to present the measured quantities in an LTE system. Whenever

the UE sends a measurement report, it also reports the measured quantities. The most used

quantities are the Reference Signal Received Power (RSRP) and the Reference Signal Received

Quality (RSRQ). Their definitions will be further described in this subsection, as well as the

relation of RSRQ with the signal-to-noise-plus-interference ratio (SINR).

The RSRP that the UE experience is an average of all the Resource Elements that contain

cell-specific Reference Signal over the entire measured bandwidth [TR36.214, 2011]. The RSRP

takes only into account the reference signal and omits the interference power and the noise. The

reporting range of the RSRP is between −140 dBm and −44 dBm. The RSRP is defined in

9


Equation 2.1 [Salo, 2013].

RSRP =1

K

K∑

k=1

Prs,k [W] (2.1)

Where Prs,k is the estimated received power for the kth Reference Signal Resource Element.

The RSRQ definition can be found in Equation 2.2 [Salo, 2013], which is the ratio between the

RSRP and the total received power and noise normalized to 1 PRB (corresponding to one slot in

time domain (1 ms) and 180 MHz in frequency domain [TR36.211, 2007]). The RSRQ indicates

the quality of the received signal.

RSRQ =Nprb

RSRP

RSSI

=Nprb

1

K

K∑

k=1

Prs,k

Nre∑

n=1

Pn

(2.2)

The RSSI (Received Signal Strength Indicator) term in the equation denotes the power from

the serving cell, the interference and the thermal noise. It contains the total power Pn observed

in the nth resource element that containing the reference signal. The total number of resource

blocks is Nre = 12 ·Nprb, where Nprb is the number of physical resource blocks in the measure-

ments bandwidth and 12 is the number of the sub-carriers. The range where the RSRQ operates

is between −19.5 dB and −3 dB [Salo, 2013].

The RSSI definition can be expressed in Equation 2.3, over the whole measurement bandwidth.

RSSI =Stot + Itot +Ntot =Nre∑

n=1

Pn [dB] (2.3)

Where S stands for received signal power measured over the 12Nprb subcarriers of the measure-

ment bandwidth, I for interference and N for noise [Salo, 2013].

In contrast with the RSRP, RSRQ is not a suitable quantity for triggering an intra-frequency

handover. This is due to the fact that the ratio between the RSRQ of the serving cell and the

RSRQ of a neighbouring cell will only depend on the ratio of the RSRPs of these cells.

Network quality can be analysed by the SINR. It can be defined in terms of RSRP, average

interference power(I) and thermal noise power (N), as in Equation 2.4.

SINR =RSRP

I +N(2.4)

10


2.2.3 Layer 1 and Layer 3 Filtering

The UE can perform inter-frequency measurements (between cells operating on two different car-

rier frequencies) or intra-frequency measurements (between cells on the same carrier frequency).

The UE perform measurements at the physical layer, Layer 1, (3GPP TS 36.214) and it reports

them to the Layer 3 (network) (3GPP TS 36.331). The 3GPP specifications contain informa-

tion about the accuracy of the measurements. Intra-frequency RSRP measurement difference

between a serving and a target cell should not have more that 3 dB error. In the case of inter-

frequency measurements, the difference between serving cell RSRP and the target cell RSRP

can be at most 6 dB. Therefore, the accuracy for the intra-frequency measurements is better

than for the inter-frequency. These accuracy specifications are part of the Layer 1 filtering of

the measurements [Salo, 2013].

In order to improve the measurements accuracy and mitigate the effects of fading , Layer 3

filtering is applied to the physical layer measurements. The raw measurements from Layer 1 are

filtered at Layer 3. The updated filtered measurement result is used for evaluating the reporting

criteria or for measurement reporting [Salo, 2013].

The Equation 2.5 shows how the filtering is done at the network layer .

Fn =(1− α) · Fn−1 + αMn (2.5)

Where:

� Mn is the latest received result from the L1 layer

� Fn is the updated filtered measurement result

� Fn−1 is the old filtered measurement result

� k is the filter coefficient for the corresponding measurement quantity

� α =(

1

2

)k

4

For inter-frequency measurements, measurement gaps can be assigned to the UE for the time

periods where no uplink or downlink transmissions are scheduled. In inter-frequency measure-

ments, when the UE has less opportunity to detect a cell, measurement gaps are configured. Two

possible measurement gaps can be configured by the network, each gap having a length of 6 ms:

in the first gap, measurements are performed every 40 ms, while in the second one they are per-

formed every 80 ms. The filtering at the two layers (1 and 3) is shown in Figure 2.4 (p. 12). The

advantage of using short measurement gap is the identification delay of a cell is shorter, but there

can be bigger interruptions in data transmission or reception [Stefania Sesia and Baker, 2011].

11


Figure 2.4: Filtering at Physical and Network Layers [Lopez-Perez et al., 2012]

2.2.4 The Handover Procedure

A detailed description of the handover procedure will be covered in this subsection, showing the

message exchange at every entity level in the LTE architecture.

There are three steps for the handover. The first one is the handover preparation, which can be

depicted in Figure 2.5. First, the eNodeB configures the UE measurements (1).

Figure 2.5: Handover Preparation [Holma and Toskala, 2009]

The decision for performing the handover is based on UE measurements and it is done by the

network. The report towards the source eNodeB is made when the measurement from the target

eNodeB has fulfilled a specific threshold condition (2). In case of intra-frequency handovers, the

UE is normally connected to the cell with the lowest path loss value because this indicates better

signal strength and lower uplink interference. The UEs that are not connected to the cell with

minimum path loss use unnecessary high transmit power, therefore they create uplink interfer-

ence. The handover request is sent from source to target eNodeB (4). An admission control

method is used for the network to check if there are enough resources for the target cell (5). If

so, the handover towards the target cell is ready to be performed, otherwise the connection will

be dropped in order to avoid high interference. The last step in the preparation of the handover

is sending an acknowledge request from the target to the source, which means that the target is

12


ready to be handed over the connection (6) [Holma and Toskala, 2009].

The second step in the handover procedure is the handover execution and it is represented in

Figure 2.6. After having received the acknowledge from the target, the source eNodeB sends

the handover command to the UE (7) and transfers the packets acknowledged by the UE to the

target through the X2 downlink interface (8). These packets are then buffered by the target

eNodeB. Using the RACH procedure (Section 2.2.5 (p. 15)) the UE synchronizes and accesses

Figure 2.6: Handover Execution [Holma and Toskala, 2009]

the target cell (9). After receiving the uplink information from the target, the UE confirms the

handover to the target, which can now send data to the UE (11).

The last part of the handover is called handover completion. Its main steps can be seen in

Figure 2.7 (p. 14). The target eNodeB has received the confirmation message from the UE and

now it informs the MME about the change of the cell. It also requests an update for the User

Plane (UP) from the S-GW, which switches the downlink data to the target. The UP update

has to be confirmed by the MME. Afterwards, the target eNodeB requests the source to release

the radio and control plane resources associated to the UE.

The reporting criteria for the measurements and the format are contained in the reporting con-

figuration. The criteria are for the UE to trigger a measurement report and they can be event

triggered or periodic. This includes quantities that are shown in the measurement report (num-

ber of cells, serving cells, listed cells). For handover measurements, it is usually the RSRP

estimation over the cells included in the list. This RSRP estimation is done at Layer 1, where

handover measurements are filtered (Section 2.2.2 (p. 9)).

A handover is triggered if the Layer 3 filtered handover measurements meets a handover event

13


Figure 2.7: Handover Completion [Holma and Toskala, 2009]

Event A1 Serving cell becomes better than absolute threshold

Event A2 Serving cell becomes worse than absolute threshold

Event A3 Neighbour cell becomes better than an offset relative to the serving cell

Event A4 Neighbour cell becomes better than absolute threshold

Event A5 Serving cell becomes worse than one absolute threshold and neighbourcell becomes better than another absolute threshold

Event B1 Neighbour cell becomes better than absolute threshold

Event B2 Serving cell becomes worse than one absolute threshold and neighbourcell becomes better than another absolute threshold

Table 2.1: Event-Triggered Reporting Criteria for LTE and Inter-RAT Mobility[Stefania Sesia and Baker, 2011]

entry condition [Lopez-Perez et al., 2012]. The types of handover event entry conditions are

presented in Table 2.1.

The eNodeB can influence the entry condition by setting the value of some parameters used in

the triggering event. These parameters are configurable and they can be one/more threshlods,

an offset and a hysteresis, depending on the event used for triggering. The parameter called

time-to-trigger is the minimum time period for which the entry condition must be satisfied in

order for an event to be triggered. [Stefania Sesia and Baker, 2011]

Figure 2.8 (p. 15) shows the triggering of the A3 event (most commonly used for handover

triggering), where the Offset and Time To Trigger are configured as threshold values. The

reporting condition is met when the signal of the neighbouring cell becomes better than the

signal of the serving cell by a certain offset. If so, the handover preparation will be performed

to the neighbouring cell after the Time To Trigger has expired and the reporting condition is

14


still fulfilled. The Time To Trigger value is set in case the serving cell signal becomes better

than the neighbouring during this time. If this happens the handover will not be performed

[Stefania Sesia and Baker, 2011].

Figure 2.8: Triggering conditions for handover in connected mode[Stefania Sesia and Baker, 2011]

The values of the configured parameters have an important role in the handover performance.

If the TTT value is too small, this may lead to handovers being performed too early, or if the

TTT is too big the handover may be performed too late. This may result in system failures

[Lopez-Perez et al., 2012].

2.2.5 The RACH Procedure

The random access procedure is used for initial network access by the UE and also in the handover

process. It can be contention-free (no collision with requests from other users in the requested

cell) or contention-based. When the UE wishes to connect to a network for the first time, it sends

a request over the medium containing the UE’s specific signature, also known as RACH preamble.

Therefore, different UEs will have different preambles, though it is possible that two UE attempt

a connection using the same preamble. In this case, there will be a collision. There are 64 avail-

able preambles for initial UE access and they are assigned differently according to the type of

RACH used: contention-based (the preamble is randomly chosen by the UE) or contention-free

(the decisions upon the preamble are made by the network) [Stefania Sesia and Baker, 2011].

When the RACH procedure is contention-free, some resources are reserved and the UE is assigned

specific resources by the eNodeB. RACH procedure is used during the handover process and the

message exchange between the UE and the target eNodeB can be seen in Figure 2.9 (p. 16). In

this case, the assignment of the preambles is signalled via a handover command from a source

to a target eNodeB, since it is the eNodeB that controls the handovers.

15


Figure 2.9: RACH Procedure in the Handover Process[Stefania Sesia and Baker, 2011]

The eNodeB requests for the target cell to prepare for the handover before sending the com-

mand to the UE in handover preparation. This can contain information about UE capabilities.

The handover command is sent from the eNodeB to the UE in the RRCConnectionRecon-

figuration message. In this messages it is included information about the cell ID, frequency

and the radio resource configuration. The measurement configuration may also be included

in this RRCConnectionReconfiguration message, but in order to avoid the message being too

large, the eNodeB can send another reconfiguration message with the measurement configura-

tion [Stefania Sesia and Baker, 2011].

If the UE is able to comply with this configuration, then it starts a timer, T304, and the ran-

dom access procedure is initiated towards the target cell. When the RACH procedure has been

successfully completed, the timer T304 stops [Stefania Sesia and Baker, 2011].

In the early releases of LTE, the handover can result in delay plus interruption time. The

interruption time is defined as the time from the end of the handover command from the serving

cell to the moment the UE starts transmitting in the uplink channel to the target cell. This

process is shown in Figure 2.10 (p. 17).

16


Figure 2.10: Interruption time in the Handover Process[Stefania Sesia and Baker, 2011]

If there are multiple frequency layers, the network may request the UE to perform measurements

on only one of the frequency layers. For the other layer the UE may perform ”blind” handovers,

meaning that it has to detect a cell before accessing it. This results in a higher interruption

time [Stefania Sesia and Baker, 2011].

The RACH procedure is also found in case there are any failures of the radio link, during the

handover or the reconfiguration procedure. In the case of failure, the UE starts the RRCConnec-

tionReestablishment procedure. For the re-establishment, the timer T311 is started and the UE

performs cell selection. When it finds a suitable cell on an LTE frequency, the timer is stopped

and the UE initiates a contention based RACH procedure, by starting another timer, T301. The

RACH procedure enables the RRCConnectionReestablishmentRequest message, which contains

information about the cell identity in which the failure occured, a message authentication and

the cause of the failure [Stefania Sesia and Baker, 2011].

This section explained how the random access procedure works when the UE first initiates

a connection, how it is used during a handover or in case there is a failure in the system.

For detailed information about the message exchange between the UE and the eNodeB, check

Section A (p. 77).

2.2.6 Mobility Failures

There are several problems that can occur in the system when trying to trigger the handover.

These will be briefly presented in this subsection, along with the causes that produce them.

The types of failures that can occur in an LTE system are as follows:

� ping-pong: back and forth handover between cells when the time the UE stays connected

to the target cell is less than a minimum amount of time (ping-pong timer, usually equal

to 1 second [TR36.839, 2012]).

17


� radio link failure (RLF): detected based on the radio link quality of the serving cell

when the UE is in RRC connected mode. The UE can be in-sync or out-of-sync with the

serving cell. The RLF is defined by a set of parameters, which can be found in Table 2.2

[Stefania Sesia and Baker, 2011].

� handover failure: it can occur in the following cases:

– a RLF occurs during the handover preparation or execution steps;

– no capacity is left in the neighbouring cells;

– handover is performed too early/late.

For tracking a radio link failure, the UE uses two channel quality indicators, Qin and Qout. A

UE is considered out of synchronisation RLF is declared if the signal-to-interference-plus-noise

(SINR) ratio is below -8 dB (Qout) and stays below -6 dB (Qin) for the duration of the timer

T310. The timer is stopped once the SINR value is larger than -6 dB. A handover failure is

declared if the timer T310 is still running when the handover command is sent or if the SINR is

still lower than Qout after the handover complete message. The process of failure detection can

be depicted in Figure 2.11.

Figure 2.11: Timers in Handover Process and RLF Detection[Lopez-Perez et al., 2012]

It may happen that before the handover command the SINR > −6 dB while the time T310

has not expired, then the system can recover from the RLF. The recovery is made through the

RRCConnectionReestablishment command, which has been described in Section 2.2.5 (p. 15).

Qin and Qout are simulation modelling parameters, while the parameters presented in Table 2.2

are real configuration parameters. The parameters are set according to the simulation setup

described in the 3GPP specifications [TR36.839, 2012].

18


Parameters Description

T310 RLF timer has expired

N310 RLF timer stops after 1 out-of-sync indication

N311 RLF timer stops after 1 in-sync indication

Table 2.2: Description of Parameters for RLF Occurrences[Stefania Sesia and Baker, 2011]

2.3 Carrier Aggregation

This section will introduce the basic concept and the motivation for using carrier aggregation.

The purpose of this section is to create a background for the techniques that will be further used

for this project. The focus of the description will be on the mobility procedures with carrier

aggregation, the performance of the system and in which networks carrier aggregation can be

utilized (heterogeneous networks).

The concept of carrier aggregation is the first step of LTE-Advanced used to optimize the net-

work efficiency, as the demand for spectrum is constantly increasing. Meeting the 150 Mbps

downlink target for LTE-Advanced requires at least 20 MHz of bandwidth. This problem is

addressed by carrier aggregation by combining multiple carriers together. The benefit from this

is that the available capacity can be used more efficiently, more users can be served and the

delay of services can be reduced.

According to Release 10 LTE specifications, carrier aggregation can be utilized for both downlink

and uplink UE transmissions. LTE-Advanced allows the aggregation of up to 5 carrier compo-

nents, each one having up to 20 MHz bandwidth. Therefore, a total of 100 MHz transmitter

bandwidth can be obtained by allocating the carriers in three different modes, depending on the

spectrum allocation: contiguous intra-band, non-contiguous intra-band and inter-band carrier

aggregation. These three modes can be depicted in Figure 2.12 [Holma and Toskala, 2012].

Figure 2.12: Types of carrier aggregation allocation in LTE-Advanced [Anritsu, URL]

The principle for the contiguous case is transmitting in two or more adjacent carriers, so the

19


aggregated channel can be seen as a single enlarged channel at the receiver end. This is pos-

sible for unpaired spectrum allocation. The non-contiguous aggregation refers to transmitting

non-adjacent carriers, but belonging to the same frequency band. The inter-band carrier aggre-

gation mode is used for transmitting two or more carriers with each of the carriers on a different

frequency band [Holma and Toskala, 2012].

Carrier aggregation can be used for both downlink and for uplink. In the case of downlink,

multiple carriers of 20MHz can be aggregated to send information to the same UE. These

carriers are received by the UE on multiple frequency bands at the same time. This is also

known as fragmentation of the spectrum. Although the principle for this design would allow

using 5 carrier components for downlink communication, the RF performance limits the number

of carriers to 2. The possible scenarios for using carrier aggregation with 2 components is

illustrated in Figure 2.13.

Figure 2.13: Downlink carrier aggregation deployment scenarios[Holma and Toskala, 2012]

The different deployments in Figure 2.13 can be using the same coverage (A), different coverage

for the two carriers (B), different latter induced by antennas and antenna tilts (C). The advan-

tage of carrier aggregation in this case is directing the second carrier to an area which is less or

not covered by the other carrier frequency, as it is presented in case C of Figure 2.13. This will

enhance the performance for the UEs at the cell edge, which are not covered by high frequencies.

Therefore, using carrier aggregation between a high and a low frequency band will improve the

performance of these users located at a cell edge, by reserving them more of the low frequency

band.

A new aspect of the eNodeB introduced by Release 10 is the concept of Primary Cell (PCell)

and Secondary Cell (SCell). The PCell is the only one to which the UE exchanges RRC sig-

nalling and and it is changed or removed during handover. One PCell is active at a time, while

more than one additional SCells can be active at the same time. PCells and SCells are both

serving cells, but the measurements and mobility procedures are based on the PCell. The acti-

20


vation/deactivation of the SCells is done at the MAC layer. Thus, the scheduling of the carrier

components is done at the MAC layer. The mobility with carrier aggregation is not changed: the

mobility measurements are done on the PCell, also known as the Primary Carrier Component

(PCC). The SCells are added, reconfigured or removed by the eNodeB through RRC Connection

Reconfiguration procedure section A (p. 77). The measurement events with carrier aggregation

are events A1, A2, A3 and A5 as they are presented in 2.1. For carrier aggregation, a new event

is introduced: A6. This informs if the neighbouring cell becomes better with an offset related

to the current SCell. The A6 event is useful for SCell intra-frequency measurements, when its

strength is very different from the strength of the PCell [Holma and Toskala, 2012].

In uplink, carrier aggregation enables the UE to transmit data on multiple carriers at the same

time. Same as in downlink, the number of carriers to be aggregated is limited to 2 (Figure 2.14).

Figure 2.14: Uplink carrier aggregation deployment [Holma and Toskala, 2012, p. 51]

The scheduler at the MAC layer in uplink carrier aggregation decides whether the UE has

enough transmit power to handle transmissions on two carriers simultaneously of it should just

transmit on one carrier. Uplink carrier aggregation is also visible to the UE through the mobil-

ity procedure and the SCells operations are also done through RRC Connection Reconfiguration.

The performance of a system with carrier aggregation in uplink differs from the downlink with

respect to the transmission power. Since the UE transmits data with less amount of power

than the eNodeB, the users at the cell edge do not gain from using carrier aggregation, since

they don’t have enough power to access the extended bandwidth. The most typical carrier

aggregation mode is C, as they are presented in Figure 2.12 (p. 19), based on the 3GPP band

combinations. The aggregated carriers can be transmitted in parallel from the same UE, thus

an increase in throughput will be obtained [Holma and Toskala, 2012]. A concrete example of a

system using carrier aggregation will be presented in section 5.3 (p. 54).

Conclusion This chapter gave an overview on the handover process in LTE, the message ex-

change between architectural entities when performing the handover, the challenges and failures

that may occur in the system and how the system can recover from these failures. Further

21


on, the following chapters will be based on this information for analysing real measurements

performance.

22

Chapter 3

LTE Performance at Low Speed

This chapter describes the performance of the operational LTE system by means of filed mea-

surements in Aalborg city center at low speed. The analysis investigates the performance in

terms of RSRP, downlink throughput and mobility performance (RLF and ping-pong effect).

Furthermore, the real scenario is reproduced by means of simulation and then compared with

the test. The comparison with the simulation is done in order to validate the model used to

generate the results.

3.1 Measurement analysis Aalborg city center

In order to analyse the performance of the LTE system measurements are performed in the

city center of Aalborg. The chosen path is shown in Figure 3.1, which was already taken into

account in [Chavarrıa et al., 2014] for a 3G study. The tests are performed using the Telenor 4G

network. The network consists of macro cells and it is characterized by two carrier frequencies:

1.8 GHz and 2.6 GHz with 20 MHz of bandwidth each.

The 1.8 GHz network has 34 sites deployed throughout the city center. The average height of

the antenna sites is 30 metres. In general, most of the sites have three sectors while some have

two or one sector. The average antenna down tilt is 3.7 degrees. The 2.6 GHz network has less

number of sites compared to the 1.8 GHz: only five. For the 2.6 GHz network the average height

is 30 metres and the downtilt average is 6.8 degrees.

Figure 3.1: Path for the measurements

23

CHAPTER 3. LTE PERFORMANCE AT LOW SPEED

3.2 Measurements campaign

For this study it is chosen to analyse separately the two frequencies, thus a total of four tests

are conducted: two for 1.8 GHz and two for 2.6 GHz. The measurements are performed during

10:00 AM and 13:000 PM by bike at an average speed of 15 km/h.

The phone used for the tests is a Samsung Galaxy III that supports the LTE tecnology at 1.8

GHz and 2.6 GHz. The UE category is 3 which supports a maximum data rate of 100 Mbps

on downlink and 50 Mbps on uplink assuming 2 x 2 MIMO system. The UE is programmed to

download 100 MB from a FTP server regularly and it waits two seconds before re-starting the

download. Furthermore, the position of the UE is recorded by using the Global Position System

(GPS). This is necessary in order to evaluate where the handovers occur. The UE is forced to

stay in one frequency at the time. This means that there are no inter-frequency measurements.

A software installed on the phone makes it possible to extract the RRC messages that the UE

exchanges with the serving eNode.

In the following sections the results for both frequencies regarding the signal level, throughput

and mobility are shown.

3.2.1 Performance at 1.8 GHz carrier frequency

This section highlights the measurements at 1.8 GHz carrier frequency for two tests along the

chosen path. In Table 3.4 are shown the minimum and maximum values for the RSRP, RSRQ

and RSSI from the serving cells found in the two tests. The table shows that the RSRP values

from the two tests are quite close as well as the RSRQ. The RSSI of the second test is only 2

dB off compared with test one.

Test RSRP (dBm) Value RSRQ (dB) Value RSSI (dB) Value

1RSRP Min.

RSRP Max

-120.1

-68.8

RSRQ Min.

RSRQ Max

-26.6

-6.5

RSSI Min.

RSSI Max

-82.7

-35.9

2RSRP Min.

RSRP Max

-120

-66.9

RSRQ Min.

RSRQ Max

-29.3

-6.9

RSRQ Min.

RSRQ Max

-80.7

-37.2

Table 3.1: Minimum and maximum values of RSRP, RSRQ and RSSI at 1.8 GHz

Since the values from the the two measurements are close on average, it is chosen to show the

RSRP value along the path only for the test one. In Figure 3.2 (p. 25), the RSRP values are

shown, together with the corresponding network layout and the antenna height. In the figure the

antenna site is depicted by a blue point from which three lines are traced. These lines represent

the orientation of each antenna. The red line is used to indicate that during the measurements

the UE is connected to the latter.

24


9.905 9.91 9.915 9.92 9.925 9.93 9.93557.04

57.042

57.044

57.046

57.048

57.05

57.052

57.054

57.056

Longitude

Latit

ude

RSRP Level at 1.8 GHz Carrier Frequency

dBm

-126.1

-106.34

-105

-85.24

-85.5

-66.8

46m

23m

34m

58m

60.5m

16.5m40.7m

50m

Figure 3.2: RSRP Level at 1.8 GHz Carrier Frequency

The minimum and the maximum values can be depicted in the color bar on the right of the pic-

ture. The minimum and maximum values of the color bar are the minimum and the maximum

RSRP values extracted from the measurements at 1.8 and 2.6 GHz carrier frequency (Table

3.4). The three ranges of values are defined in red, black and green, from bad signal strength

(minimum) to very good signal strength (maximum).

From the figure it is possible to see that the RSRP reaches high levels (green zone) in several

areas mostly where the UE is closer to the BS. The lowest values of RSRP are found in the

intersection of the streets. This indicates a poor coverage in that area.

From Figure 3.2 it can be concluded that in general the signal strength that the UE receives

is good, but this is not enough to evaluate the performance of the system. Another important

aspect to take into account is the data-rate that the UE experiences as well as the mobility

performance. In the following sections these aspects are investigated.

Throughput

In Figure 3.3 (p. 26) the CDF of the throughput is shown relative to the test in Figure 3.2. The

peak data-rate value is reached at 26.3 Mbps, while the 80 % of the time is below 10 Mbps.

25


0 5 10 15 20 25 300

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Mbps

CD

F

Throughput at 1.8 GHz

Figure 3.3: CDF Throughput at 1.8 GHz

By comparing the maximum throughput that the phone can support (100 Mbps) and the average

throughput from the measurements (8 Mbps) it can be noticed that the performance in real life

is far away from the theoretical assumption. The UE could get only the 20% of the theoretical

throughput (100 Mbps).

Mobility

For this measurements the mobility aspect is also investigated: the handover performance in

the system is related to the radio link failures and the ping-pong effect. In order to understand

how the mobility works and which event triggers the handover, the message exchange between

the UE and the eNodeB is analysed (see Appendix A). The information extracted from the

messages highlights that the event that triggers the handover is the A3 event. For this event

two different set of parameter values are found. This means that the handover is triggered with

two different configurations of A3. The set of values used for triggering the handover are shown

in Table 3.2. Most of the cells where the phone is connected to during the measurements use

A3 Configuration 1 Value A3 Configuration 2 Value

TTT

Offset

Hysteresis

1024 ms

2 dB

2 dB

TTT

Offset

Hysteresis

1280 ms

2 dB

2 dB

Table 3.2: Different A3 event configuration

Configuration 1. Configuration 2, the one with a larger TTT, is used by the cell beyond the

fiord (see Figure 3.2 (p. 25)). The reason for this choice might be related to the position and

the antenna height of this specific cell. Even though the antenna is placed far away from the

city the UE receives signal from it. The large TTT value ensures that the signal strength of

26


the target cell remains better than the signal strength of the serving cell for a bigger amount of

time. This would prevent an early handover.

Under these circumstances, the number of handovers performed by the UE for Test 1 are 1.58 per

minute. No RLFs are detected during the measurements. The reason for this is related to the

value of the timer T310. This timer is used to allow the UE to get back in synchronization with

the BS. If this timer expires and the phone is still out of synchronization, the RLF is declared.

By analysing the Layer 3 messages the value of T310 is found to be 2 seconds. With this large

value, the UE has enough time to go back in synchronization with the BS.

To calculate the ping-pong effect (PP), the 3GPP recommends the ping-pong timer to be 1

second [TR36.839, 2012]. In this case, the latter timer value can not be applied to this mea-

surement. This is because the TTT (Time To Trigger) value for the A3 event is more than 1

second. This means that the UE has to be connected to the cells at least the TTT timer. Due

to this, the ping-pong timer has been chosen to be 1.5 seconds. In this study, according to this

value no ping-pongs are detected on the simulations as well as radio link failures.

Test HOs/min

Test 1 (∼15 km/h)

Test 2 (∼15 km/h)

1.58

1.75

Table 3.3: Number of HO, RLF and PP per minute at 1.8 GHz

Table 3.3 summarizes the number of handovers per minute occurred in the all tests. The two

measurements show on average 1.66 handovers per minute.

3.2.2 Performance at 2.6 GHz carrier frequency

The same measurements are done for 2.6 GHz. In this scenario the number of antennas is less

compared to the 1.8 GHz network. As in the case of 1.8 GHz the signal level, throughput and

mobility performance are presented.

In Table 3.4 the minimum and maximum values of RSRP, RSRQ and RSSI obtained from the

two measurements are shown.

Test RSRP (dBm) Value RSRQ (dB) Value RSSI (dB) Value

1RSRP Min.

RSRP Max

-125.7

-68.20

RSRQ Min.

RSRQ Max

-19.1

-5.9

RSSI Min.

RSSI Max

-92.3

-37.9

2RSRP Min.

RSRP Max

-126.1

-67.5

RSRQ Min.

RSRQ Max

-20.1

-6.2

RSRQ Min.

RSRQ Max

-92.6

-39.4

Table 3.4: Minimum and maximum values of RSRP, RSRQ and RSSI at 2.6 GHz

By comparing these values with those obtained in 1.8 GHz (see Table 3.4) it can be noticed that

27


in general the signal in 2.6 GHz scenario is 5 dB weaker. Figure 3.4 shows the RSRP level for

the Test 1, since there is not much difference between the two tests. On top it is also plotted the

network layout, where the BS which the UE is connected to are marked with a red line. As in

the case of 1.8 GHz, the lowest signal is found in the junction. This is due to lack of coverage in

that area. In general the signal strength level (RSRP) is quite satisfying, as well as the RSRQ

and RSSI values.

9.9 9.905 9.91 9.915 9.92 9.925 9.93

57.042

57.044

57.046

57.048

57.05

57.052

57.054

57.056

57.058

Longitude

Latit

ude

RSRP Level at 2.6 GHz Carrier Frequency

dBm

-126.1

-106.34

-105

-85.24

-85.5

-66.8

34m

28m

28m

Figure 3.4: RSRP Level at 2.6 GHz carrier frequency

Throughput

Figure 3.5 (p. 29) shows the CDF of the throughput at 2.6 GHz relative to the measurement in

Figure 3.4. The peak is reached at 16 Mpbs while on average the UE experiences 7.5 Mbps. The

throughput peak value for this measurement is lower that the peak obtained for 1.8 GHz. The

reason for this can be related to the unoptimized coverage, since the number of sites deployed

in the 2.6 GHz network is limited.

28


0 2 4 6 8 10 12 14 160

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Mbps

CD

F

Throughput at 2.6 GHz

Figure 3.5: Throughput CDF at 2.6 GHz

Mobility

Also in this case, by analysing the Layer 3 messages, it is found that the event that triggers the

handover is event A3. For this event two different configuration are also found, as the Table

3.5 shows. The choice of having different configurations might be for avoiding too early/late

handovers.


TTT

Offset

Hysteresis

1024 ms

2 dB

2 dB

TTT

Offset

Hysteresis

320 ms

3 dB

3 dB

Table 3.5: Different A3 event configuration

The number of handovers per minute during this measurement campaign is 0.91. Similarly to

the case of 1.8 GHz no RLF are detected as well as ping-pongs. The reason for this is already

explained in Section 3.2.1. Table 3.3 (p. 27) summarizes the handovers, RLF and ping-pong per

minute occurred in all the performed measurements.

29


3.3 Simulation versus real measurements

In the telecommunication field simulations play an important role, since the introduction of new

features in the system needs to be tested before the commercialization. For this reason the

comparison between the performance obtained in the real measurements and the simulation is

presented in order to verify the reliability of the model used to represent the reality. In the

following it is also described how the simulation methodology is working as well as the utilized

model.

3.3.1 Simulation methodology and model environment

A MATLAB based dynamic system level simulator is used to simulate a cellular network char-

acterized by multiple cells and multiple users. The users are placed in the network and they are

moving at constant speed.

In order to replicate the scenario, a 3D map of the city center of Aalborg is used. The map con-

tains 3D data for streets and buildings, as well as open area and the fiord. The path loss map of

the area is computed by using ray-tracing techniques based on the Dominant Path Model. The

path loss map also takes into account the shadowing computation. The propagation is assumed

to be constant within a 25m2 metres (5m x 5m).

The antenna sites in the network are placed according to the data provided by the operator,

as well as the height and the antenna tilt. In the simulation the users are following the same

path as in reality and they are moving with a constant speed of 15 km/h. In addition, the users

are uniformly distributed along the path. The used traffic model is full buffer where the UEs

are transmitting all the time. This choice replicates the reality of the measurements, where the

phone was downloading a file all the time from a FTP server.

In Table 3.6 (p. 31) are shown the parameters used for the simulation at 1.8 GHz and for 2.6

GHz. The values of the A3 event (TTT, Offset and Hysteresis) are taken from the values found

in the messages for each frequency as explained in Section 3.2.1 (p. 24) and Section 3.2.2 (p. 27).

The Qout and Qin are the SINR values for the entering condition and leaving condition of the

radio link failure. Meaning that, when the SINR is below -8 dB the timer T310 starts. If after

this time the SINR is below -6 dB a RLF is declared. The T310 value is set according to the

value found in the Layer 3 messages. All these parameters together with the created propagation

maps are loaded in the simulator.

30


Parameters 1.8 GHz Settings 2.6 GHz Settings

Scenario Setup:

- Carrier Frequency

- Bandwidth

- PRB Number

- DL Transmit Power

- Propagation Model

- Antenna Height

- Traffic Model

1.8 GHz

20 MHz

100

46 dBm

Ray tracing DPM

According to the reality

Full buffer

2.6 GHz

20 MHz

100

46 dBm

Ray tracing DPM

According to the reality

Full buffer

Handover Parameters:

- HO Trigger Event

- TTT

- Hysteresis

- Offset

A3 Event

1024 ms or 1280 ms

2 dB

2 dB

A3 Event

1024 ms or 320 ms

3 dB

3 dB

Failure Detection Parameters:

- RLF Qout

- RLF Qin

- T310

-8 dB

-6 dB

2 sec

-8 dB

-6 dB

2 sec

User setting:

- Num. of users in the street

- Distribution of the users

- User speed

- Background users

300

Uniform

15 km/h

None

300

Uniform

15 km/h

None

Table 3.6: Simulation Parameters at 1.8 GHz and 2.6 GHz

3.3.2 Simulation at 1.8 GHz

By using the parameters in Table 3.6, this section will be shown the outcome from the simulation

at 1.8 GHz carrier frequency and compare it with the real test. In Figure 3.6 (p. 32) it is shown

the handover position from the measurements at 1.8 GHz carrier frequency. The area where

the handover occurs is marked with a black ellipse. In order to see if the the outcome from the

simulation reflects the reality, in Figure 3.7 (p. 32) the position of the handovers is shown.

31


Figure 3.6: Handover position 1.8 GHz from the measurement

-1200 -1000 -800 -600 -400 -200 0

-400

-200

0

200

400

600

800

4

5

7

16

17

18

23

35

46

50

51

52

68

6970

74

75

76

lotitude

lang

itude

Handover Position

Base Stationantenna orientation

Figure 3.7: Handover position 1.8 GHz from the simulation

In Figure 3.7 the blue dots represent the position where the handovers occur in the simulation.

By comparing the two figures is possible to see that the simulation matched quite well with

the measurements. Only one discrepancy with the reality is found. This is marked by an red

ellipse in Figure 3.7. The reason for this is related to the resolution of the propagation map,

where within 25m2 the signal is constant. In fact, due to the resolution, some street pass close

to buildings. Also the signal strength is lower due to the penetration loss close to buildings. In

addition, the calculation of the signal in a certain point is made by taking the interpolation of

the values of the surrounding point. Thus, if the distance between the street and the building

32


is low in that point the signal strength will be low too. So it might not be possible to replicate

the same conditions as in the reality. From the point of view of the RLF the result from the

simulations matched with the reality. The number of handovers per minute is 3.1 which results

to be the double compared with the value obtain from the reality (1.58 on average).

3.3.3 Simulation at 2.6 GHz

In this section the results from the simulation at 2.6 GHz are shown by taking into account

the parameters in Table 3.6. In Figure 3.8 it is shown the handover position obtained from the

measurement marked with a black ellipse and in Figure 3.9 (p. 34) it can be seen the handover

position that the users experience from the simulation, which is represented by blue dots.

By comparing the two figures, it can be noticed that in the simulation there is an area where the

handover does not occur in the reality. This area is marked with a red ellipse in Figure 3.9 (p. 34).

This is due the presence of a ”coverage island” from another cell. The number of handovers per

Figure 3.8: Handover position from the measurement at 2.6 GHz

minute that occur in the simulation is 1.96. This value results to be higher compared to the one

obtained from the reality (0.91 on average).

33


-1200 -1000 -800 -600 -400 -200

-1000

-500

0

500

1000

1

2

3

4

5 7

8

9

10

11

12

15

Y [m

]

Handover Position from the simulation at 2.6 GHz

X [m]


Figure 3.9: Handover position from the simulation at 2.6 GHz

3.4 Discussion

For this particular scenario no RLFs are found. Different result are found in [Chavarrıa et al., 2014]

for 3G system, where RLFs are detected. The absence of RLF is justified by the use of a large

TTT (more than one second) and a larger timer T310 (2 seconds) which in charge of declaring a

RLF when it expires.

From the point of view of the throughput, in average the throughput for the both frequency is

found to be (8 Mbps) instead the peak was higher for the 1.8 GHz (26 Mbps) than the 2.6 GHz

case (16 Mbps). The difference in the peak values is due to the coverage, since for the 1.8 GHz

case the deployed network was dense compare with the 2.6 GHz case. Another reason might be

related to higher interference.

By analysing the simulation and making a comparison with the reality, it is possible to say that

the simulation matched quite well the result from the test. In fact, the area where the handover

occurred in the reality it could be replicated in the simulation, even though some problems relate

to the resolution of the used propagation map are detected. Also, the number of handovers in

the simulation for 1.8 GHz case is larger than the one for the 2.6 GHz reflected the reality.

34

Chapter 4

On the way towards 5G

This chapter highlights the mobility performance of the LTE system in a high speed scenario.

The focus is on evaluating the mobility failures and the throughput received by the UE. Sim-

ulations are performed in order to recreate the scenario from the measurements. It is wanted

to prove that in this scenario the current LTE system cannot satisfy the increased request for

data (10000 times more traffic) [Nokia, 2015a], therefore the simulations are used as a baseline

for implementing other technologies. For the purpose of this thesis the assumption for the 5G

are taken into account. In fact the 5G will need the use dense network, high frequency and dual

connectivity with small cells and macro cells.

The first section of the chapter describes the measurements performed on the highway in Aalborg.

The next sections will give an insight of the heterogeneous networks concept, which will be the

baseline for creating a new scenario that will be introduced later in the following chapter. The

last section in this chapter gives an overview of the targets for the 5G technology and some

study proposals will be taken as an example for an insight of what this technology is expected

to offer.

4.1 Measurement analysis Aalborg highway

After having analysed the data from the measurements in the city center of Aalborg at a relative

low speed and verified that at that specific speed the LTE system is quiet reliable in terms of

mobility performance, it is essential to see if increasing the speed of the UE the system is

still stable. In this section, the outcome from the measurements on the highway in Aalborg

are presented. The measurements were performed using the same setup as in the previous

measurements in the city center (Chapter 3). Due to the GPS it is possible to record the position

of the UE and evaluate where its connectivity in certain positions. The same parameters as in

the previous measurements are evaluated: RSRP, downlink throughput, mobility performance

and system failures. Afterwards, the scenario will be reproduced with simulations and their

outcome will be analysed. As Figure 4.1 (p. 36) shows the measurements are taken from exit 26

to exit 39 in the highway E-45.

35

CHAPTER 4. ON THE WAY TOWARDS 5G

Figure 4.1: Path for the measurements

The chosen route has been travelled at two different speeds: 80 km/h and 100 km/h. In total 8

measurements have been taken: 2 from point A to point B (Figure 4.1) and 2 vice versa from B

to A, all four of them at 80 km/h; the same has been done for 100 km/h.

The measurements will be analysed only for the 1.8 GHz carrier frequency, since it is the only

frequency provided by the network in this specific area. Therefore the phone is forced to stay in

1.8 GHz carrier frequency during the drive tests (intra-frequency measurements).

� Measurements at 80 km/h

First, the measurements at 80 km/h will be analysed. The minimum and maximum values

of the RSRP, RSRQ and RSSI from the tests are shown in Table 4.1 (p. 37). To show how

the system performs at high speed, Test 1 has been chosen to represent the path A-B and

Test 2 for the path B-A.

36


Drive Test RSRP (dBm) Value RSRQ (dB) Value RSSI (dB) Value

1 A-BRSRP Min.

RSRP Max

-126.3

-68.7

RSRQ Min.

RSRQ Max

-24.8

-6.3

RSSI Min.

RSSI Max

-87.1

-40

2 B-ARSRP Min.

RSRP Max

-122.6

-74.6

RSRQ Min.

RSRQ Max

-20.7

-6.3

RSRQ Min.

RSRQ Max

-83.1

-47.1

3 A-BRSRP Min.

RSRP Max

-125.5

-69.3

RSRQ Min.

RSRQ Max

-24.9

-5.6

RSRQ Min.

RSRQ Max

-85.4

-37.6

4 B-ARSRP Min.

RSRP Max

-132.1

-72.5

RSRQ Min.

RSRQ Max

-26.8

-5.5

RSRQ Min.

RSRQ Max

-85.4

-45.4

Table 4.1: Minimum and maximum values of RSRP, RSRQ and RSSI obtained for 80km/h

Figures 4.2 and 4.3 show the RSRP values for the route A-B and for B-A, respectively. As

in the previous measurement campaign, the minimum and maximum values of the RSRP

can be depicted in the color bar, from bad to very good signal strength. Once again, the

three ranges of colors have been chosen to represent the signal: red, black and green, as

shown on the bar. In order to evaluate the signal changes along the path and be able to

compare the two routes, the same ranges for the three colors have been chosen. These can

be observed on the color bar.

9.9 9.92 9.94 9.96 9.98 10 10.0257.01

57.02

57.03

57.04

57.05

57.06

57.07

57.08

57.09

Longitude

Latit

ude

RSRP Level at 80 km/h Path A−B

dBm

-126.3

-107.1

-108

-88.8

-89

-68.7

Figure 4.2: RSRP Level at 80 km/h for Path A-B

37


9.9 9.92 9.94 9.96 9.98 10 10.0257.02

57.03

57.04

57.05

57.06

57.07

57.08

57.09

57.1

Longitude

Latit

ude

RSRP Level at 80 km/h Path B−A

dBm

-122.6

-106.6

-107

-91

-92

-74.6

Figure 4.3: RSRP Level at 80 km/h for Path B-A

As it can be noticed, the RSRP for the two paths is quite similar, apart from some small

differences. Most of the values are above -106 dBm, marked in black, indicating a good

signal strength. Some areas, like the beginning and the end of the route have very good

signal strength (indicated with green) and the areas with poor signal strength (red) are

very few.

Throughput The throughput for the two routes is shown in Figure 4.4 (p. 39). For

both paths, almost 65% of the time the throughput is below 10 Mbps and the peak data

rate is around 19 Mbps.

38


0 2 4 6 8 10 12 14 16 18 200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Throughput [Mbps]

CD

F

Path at 80 km/h

A-BB-A

Figure 4.4: CDF Throughput at 80 km/h for both paths

The average throughput found in the measurements is 9 Mbps for the two routes. As in

the previous drive tests at low speed, this result is far away from the maximum throughput

of 100 Mbps that the phone can support.

� Measurements at 100 km/h

The second set of tests has been conducted at 100 km/h, following the same paths as

before in order to see how increasing the speed would affect or not the performance of the

LTE system. The RSRP, RSRQ and RSSI values have been collected from these tests and

they are shown in Table 4.2.

Drive Test RSRP (dBm) Value RSRQ (dB) Value RSSI (dB) Value

5 A-BRSRP Min.

RSRP Max

-122.7

-69.5

RSRQ Min.

RSRQ Max

-23.2

-6.1

RSSI Min.

RSSI Max

-81.6

-40.6

6 B-ARSRP Min.

RSRP Max

-115.9

-61.3

RSRQ Min.

RSRQ Max

-29

-5.9

RSRQ Min.

RSRQ Max

-76.2

-34.8

7 A-BRSRP Min.

RSRP Max

-118.1

-63.6

RSRQ Min.

RSRQ Max

-23

-6.1

RSRQ Min.

RSRQ Max

-80.3

-35.9

8 B-ARSRP Min.

RSRP Max

-114.6

-68.8

RSRQ Min.

RSRQ Max

-25.7

-6.1

RSRQ Min.

RSRQ Max

-77.1

-38.7

Table 4.2: Minimum and maximum values of RSRP, RSRQ and RSSI obtained for100 km/h

Once again, one test representing each path was chosen for the system evaluation. The

values of the RSRP along the two paths are presented in Figure 4.5 and Figure 4.6.

39


9.9 9.92 9.94 9.96 9.98 10 10.0257.02

57.03

57.04

57.05

57.06

57.07

57.08

57.09

57.1

Longitude

Latit

ude

RSRP Level at 100 km/h Path A−B

dBm

-122.7

-105

-106

-88.3

-89

-69.5

Figure 4.5: RSRP Level at 100 km/h for Path A-B

9.9 9.92 9.94 9.96 9.98 10 10.0257.02

57.03

57.04

57.05

57.06

57.07

57.08

57.09

57.1

Longitude

Latit

ude

RSRP Level at 100 km/h Path B−A

dBm

-115.9

-97.7

-98

-79.8

-78

-61.3

Figure 4.6: RSRP Level at 100 km/h for Path B-A

The figures show that compared to the tests at 80 km/h, the values are slightly greater

at 100 km/h, as it can be noted on the color bar to the right side of the figure. Most of

the values along the path are above the average, since they are represented in black and

green, meaning that the overall signal strength is good. Next, it is wanted to check if high

speed would affect the system in terms of received throughput and mobility performance.

40


Throughput Figure 4.7 shows the cumulative distribution function of the throughput

extracted from the two measured paths. The peak data rate obtained is 20 and 23 Mbps.

The average throughput is also analysed and it is again found to be 9 Mbps, as in the

measurements of 80 km/h, still very different from the theoretical assumptions.

0 2 4 6 8 10 12 14 16 18 20 220

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Throughput [Mbps]

CD

F

Path at 100 km/h

A-BB-A

Figure 4.7: CDF Throughput at 100 km/h for both paths

4.1.1 High Speed Mobility

Failures in the system are an important aspect in the mobility procedure, as it has been previ-

ously discussed in Section 2.2.6 (p. 17). Therefore, radio link failures, handovers and ping-pong

effect will be further analysed for both speeds used for the drive tests.

For this chosen path on the highway, different configurations have been assigned to the cells

regarding the TTT, offset and hysteresis values. As in the measurements analysis from the city

center, it has been found that the handover is triggered by the A3 event and the cells that the

UE connected during measurements are configured with different values of the event. These are

shown in Table 4.3. Most of the cells have Configuration 1, while only two others are configured

differently. As for most of the cells the TTT value is more than 1 second, it can be stated

again that the ping-pong timer should be set to 1.5 seconds in order to be able to analyse the


TTT 1024 ms TTT 320 ms

Offset 2 dB Offset 2 dB

Hysteresis 2 dB Hysteresis 2 dB

Table 4.3: Cells configured with different A3 event parameters

41


ping-pong effect. Using this value of 1.5 seconds, no ping-pong effect has been noticed.

Even though the speed of the UE was higher during these sets of measurements compared to

those made in the city center, only one RLF has been detected by analysing the Layer 3 mes-

sages. This failure is irrelevant given the number of measurements. This is a really good result

since there is also a tunnel that goes under the fiord (600 metres long): while the UE is passing

through the tunnel the connection never dropped, only paging messages are recorded.

Simulations are made to model the scenario from the measurements. The simulations are based

on the highway scenario, where the real network from the measurement is taken into account,

meaning that the position of the antennas is based on the data provided by the operator, as well

as the highway position and shape.

4.1.2 Simulation for High Speed Mobility

The simulation methodology used to perform simulation of the scenario for high speed mobility

is different to the one described in Section 3.3.1 (p. 30). In fact, in this case, the 3D map used

to generate the path loss model for the simulation in Aalborg city center is not available. This is

because the position of the highway result to be outside the borders of the map. For this reason,

is necessary to use another propagation model.

For this simulation, most of the parameters for the 1.8 GHz carrier frequency are the same as

in Table 3.6, except for the propagation model and the TTT values of the cells, which can be

found in Table 4.3 (p. 41). One drive test (measurement) described previously has shown the

movement of the UE from point A to point B and another test was taken from B to A and so

on. The simulations for this scenario are performed a bit differently, since the movement of the

users in not made separately from A to B and from B to A as in the drive tests. But instead,

the movement is simulated an entire circular path, where the user departs from point A and

finishes its route in the same point, so the whole path is covered in one simulation. The layout

of the streets and network used for the simulation is shown Figure 4.8 (p. 43).

42


-2000 -1000 0 1000 2000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

Streets and Network Layout

X [m]

Y [m

]

1

2

3

4 5

6

7

8

9

10

11

12

13

14

15

16

17

1819

20

2122

23


Figure 4.8: Macro Network at 1.8 GHz and Streets Layout in Simulations for 100km/h

Path loss model The network for simulation of high speed mobility is made of 23 macro

cells, with their coordinates taken from the real measurements. Also the street model has been

created using the real geographical coordinates with two lanes on each side. The propagation

model used for this simulation is expressed in equation 4.2 according to the 3GPP standard

([TR25.942, ]):

PL =40 · (1− 4 · 10−3Dhb)log10(R)− 18log10(Dhb) + 21log10

(

fc

MHz

)

+ 80 [dB] (4.1)

Where:

� R is the distance between the BS and the UE

� Dhb is the antenna hight in metres from the average rooftop

� fc is the carrier frequency

Since in the reality the antennas have different heights, while the model above takes as an input

only one value for the antenna height, it is important to check if the model fits with the scenario.

In order to do that, the average antenna height from the reality has been found: 30 metres. The

path loss model considers the antenna height from the rooftop so it is needed to scale the real

antenna height according to that. It is assumed that the buildings are 3 storeys high with 3

meters per floor. Thus, each building is approximated 10 meters high. This leads to having an

antenna height from the rooftop equal to 20 meters. Thus, using 20 meters antenna height and

1.8 GHz as carrier frequency (according to the measurement), the equation 4.2 becomes:

PL =124.94 + 36.8 · log10(R) [dB] (4.2)

43


The antenna gain value is found in the data sheet relative to the model antenna used ([Kathrein, 2009]),

the frequency and bandwidth used are taken from the measurements as well as the values of

event A3 (TTT, Offset and Hysteresis). A full buffer traffic model is also considered, meaning

that the users always have unlimited amount of data to transmit. The transmission of their data

never ends and therefore this model allows a simplified analysis of the simulation. An overview

of the parameters used in the simulation is shown in Table 4.4.

Parameters Setting

Scenario Setup:

- Carrier Frequency

- Bandwidth

- PRB Number

- DL Transmit Power

- Macro Path Loss Model

- Antenna Height

- Antenna Gain

- Number of sites

- Network Type

- Traffic Model

1.8 GHz

20 MHz

100

46 dBm

124.94 + 36.8 · log10(R)

20 meters from the rooftop

17.5 dBi

23

4G Macro

Full buffer


- HO Trigger Event

- Time To Trigger (TTT)

- Hysteresis

- Offset

A3 Event

1024/0.320 ms

2 dB

2 dB

Failure Detection Parameters:

- RLF Qout

- RLF Qin

- T310

- Time PP Detection

-8 dB

-6 dB

2 sec

1.5 sec

User Setting

- Num. of user in the street

- Distribution of user

- User Speed

- Background users

- Speed of background users

200

Randomly placed along the street

80/100 km/h

None

None

Table 4.4: Parameters used for the high speed mobility simulation

Further on, the simulated LTE system is pushed to have minimum data-rate of 10 Mbps and

then it is verified how many users in the system are getting the minimum required.

44


Simulation Outcome The outcome from the simulation is related to the handovers, failures

in the system and achieved data rate. Table 4.5 shows the number of handovers per minute both

in real measurements and in simulations.

Speed (km/h)HOs/min

Real Measurements

HOs/min

Simulations

80 9.3 1.86

100 7 2.31

Table 4.5: Number of handovers per minute in drive tests and simulations

There is a big discrepancy between the results found in simulations and the real drive tests. The

reasoning for this can be that in the drive tests, due to the variations in the radio propagation of

the signal (surrounding cars, trucks movement), additional handovers take place. These are not

reproduced in the simulator. Furthermore, in the Table 4.5 the numbers of the handovers from

the measurements are the sum of the handovers obtained from the point A to B and vice versa.

In the simulation the entire path is covered will all the users. So, it is more correct compare the

number of the handovers for each path (from A to B or B to A) instead that with both. In fact,

if the number of handover per minute for the measurements in Table 4.5 is divided by two it get

closer to the handovers per minute in the simulation.

The presence of the tunnel in the real measurements is another aspect which is not considered

in the simulations. Therefore, the simulations for this scenario do not match the measurements

because there are handovers in the area where the tunnel exists in reality.

However, no radio link failures or ping-pongs have been found in the simulations. From this

point of view, the simulations match the reality.

After having proved that the simulation outcomes do not alter the reality, a step forward is

needed. Figure 4.9 (p. 46) shows the throughput obtained from the simulation. The figure shows

that 30 % of the time the UEs are experiencing 1 Mbps but most of them are not achieving the

minimum required (10 Mbps) for this project.

45


10-2

10-1

100

101

102

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Throughput [Mbps]

CD

F

Received Throughput

80 km/h1 Mbps100 km/h

Figure 4.9

Therefore, it is wanted to find a way of improving this result, which means implementing a

different kind of scenario. This will be discussed in Chapter 5.

Conclusion By analysing the outcome of the measurements on the highway it can be con-

cluded that LTE is stable and reliable for this particular scenario. It is resilient to radio link

failures and ping-pong effect even at high speed. But LTE can not guarantee to support the

requested volume of data to come in the next few years. Therefore, new technologies and new

methods have to be investigated to have more capacity available. For this reason, the next section

will introduce the approach for heterogeneous networks and how it can provide the increasing

number of users with their requested data volume.

4.2 Heterogeneous Networks approach

The rapidly growth of users using mobile broadband has lead the researchers to find new solu-

tions to support the demand of high data rates and system capacity. For this reason the LTE

system is constantly evolving. The fist release of LTE specifications, Release 8, was completed in

2008, which provides downlink peak data rate of 100 Mbps within a 20 MHz downlink spectrum

allocation [TR25.913, 2008]. The Release 9 introduces the concept of heterogeneous networks

(HetNet), which means the introduction of small cells on top of the macro layer. The Release 10,

known as LTE-Advanced, proposes a method to increase the capacity of the system using spec-

trum flexibility (carrier aggregation) that allows to reach a peak data rate of 1 Gbps for downlink.

46


The HetNet configuration plays an important role for the future technology because increasing

the number of small cells will result in an increase of capacity per area. This new network

topology will bring challenges from the mobility point of view since the UE may trigger several

handovers when passing through the small cells. Under these circumstances, the UE speed has

a crucial role in the mobility performance. In a HetNet scenario, using the same TTT value

for all the cells (macro and small) may deteriorate the mobility performance. For this rea-

son is needed to specify different values of TTT according to the events specific for each layer

(macro and pico) [Lopez-Perez et al., 2012].Since the next-generation will need to coexist with

the actual deployment the HetNet configuration is chosen in order to be closer to the reality.

As mentioned above, carrier aggregation is one of the new features of LTE-Advanced. With

its introduction it is possible to increase the data rate enormously. For example, drive test

have been performed by Nokia Networks. In this test 10 + 10 MHz aggregation is used. Using

only 10 MHz the data rate is around 30 Mbps, but it doubles if the carrier aggregation is used:

60 Mbps. The more bandwidth is allocated the more throughput the UE will get [Nokia, 2015b].

Due to the benefit of carrier aggregation together with the HetNet configuration the combination

of the two is used in this thesis, by focusing on the mobility design. For the macro layer the real

highway network is considered with the information provided by the operator and measurements

(Section 4.1 (p. 35)) and for the small cells it is applied the concept of 5G. An overview of what

5G will be is described in the next Section 4.3.

4.3 5G overview

As it has been proved in the previous chapter, it is not possible to achieve the minimum of 10

Mbps per UE in a high-speed environment with LTE. For the goal of this thesis, it is essential to

look for a new technology. Lately the interest of many engineers and researchers goes towards

5G. So far there is no clear explanation of what exactly the 5G is, but it is possible to identify

the challenges that would lead to its development.

The aim of 5G is to support the growth of the data volume, the increasing number of devices

and the request of high-speed data rate. In order to achieve this it is the bandwidth should

increase and the network type will gravitate towards an ultra dense network (UDN). The main

changes will be further discussed and they are based on [Jeffrey G. Andrews et al., 2014]:

� High Data Rate: the aim of 5G is to support the great request of data. It is expected that

the total amount of data rate would increase 1000 times from the current 4G technology,

with the implementation of UDN, increasing the bandwidth and advancing in MIMO

techniques.

� Bandwidth: in order to increase the bandwidth it is necessary to increase the carrier

frequency. This is because the spectrum from hundreds of MHz up to few GHz is already

occupied by different technologies. Until now is not clear what it will be the specific range

47


of frequencies, but for example METIS (Mobile and wireless communications Enablers for

the Twenty-twenty Information Society - European project that lay the foundation for the

5G) is actually investigating up to 90 GHz.

� Ultra Dense Network: having an UDN is also one of the important features. This is

due to the usage of high frequencies. An important benefit is that using small cells will

allow to reuse the spectrum more often, there would be less users per cell, each base station

being able to serve less users and therefore the traffic will be more bursty.

� Multi-RAT: the concept of heterogeneous networks will become more frequent on the way

towards 5G, since their benefit is to reduce coverage holes, minimize the distance between

the transmitter and receiver, especially for very populated areas. Multiple RATs will be

combined in order to support 5G technology together with 3G or LTE. For this type of

implementation, there are challenges regarding the load for each base station, users being

able to switch base stations according to the gains to those specific base stations and the

load.

This project will focus mainly on achieving high data rate. For this study concepts like UDN,

increased bandwidth will be implemented, which have been briefly introduced. Other topics like

millimeter waves and massive MIMO are presented in [Jeffrey G. Andrews et al., 2014]. The

main idea of the millimeter waves is utilizing the idle spectrum where wavelengths are about

1-10 mm, requiring many antennas to steer the beam. Concerning the technique of massive

MIMO, it refers to equipping the base stations with a very large number of antennas, which

would help focusing signals on very small regions of space. These two concepts will not be

further analysed, since they do not make the subject for the study.

As the studies for a new technology are progressing, each concept comes with its own imple-

mentation challenges. Mobility in a very dense heterogeneous network is difficult to model and

analyse as additional layers at different frequencies are being introduced. For this reason, multi-

ple studies have been conducted in order to find methods for improving the mobility performance

in such a system.

For example, one of the studies presented in [Ishii et al., 2012], proposes a method of splitting

the control plane (C-plane) and the user plane (U-plane) of the radio link in a HetNet scenario

consisting both of macro and small cells. The C-plane is defined at the macro layer, which

operates at a small frequency because of the need for reliable coverage (improved mobility),

while the U-plane is set for the high frequency small cells layer, resulting in improved data rate.

Another concept presented in the study is the Phantom cell. This is the name given for the small

cells, since they are only used for carrying user traffic and are not configured with cell specific

parameters. The basic mobility performance is obtained as the macro cell manages the RRC

signaling between the UE and the phantom cell. As a conclusion, improvements in capacity and

cell edge users data rates will be obtained.

Another study conducted by [Song et al., 2014] introduces the signal-to-interference ratio as the

48


handover trigger quantity. This study is made for high speed scenarios and handover occurrences

are predicted by taking into account the N+1 samples of the measured quantity. This type of

prediction is called Grey system. The predicted values are used for triggering the handover,

resulting in improved handover success probability.

This project also approaches mobility studies in a high speed HetNet scenario, as some of the 5G

elements discussed in this chapter are introduced in the scenario. The detailed implementation

and the utilized parameters will be further presented in Chapter 5, along with the results that

will be obtained.

49

Chapter 5

5G Implementation

As it has been stated in the previous chapters the need for satisfying the growing number of sub-

scribers and the increase in mobile data traffic adds a great complexity to the wireless networks.

The challenge is to optimize the network efficiency given that there are many different technolo-

gies to manage (LTE on top of GSM and HSPA, for example), thus there is a trade off between

quality and cost effectiveness. The purpose of this chapter is to present the implementation of

a new scenario whose target is to manage mobility through two different network technologies

in order to satisfy the minimum required data rate of 10 Mbps per UE.

In the first section the network deployment will be shown, which is a HetNet consisting of macro

cells and small cells that operate in frequencies belonging to two different radio access technolo-

gies. The emphasis will be on the small cell deployment according to the 5G target.

The next sections of the chapter will describe the prameters chosen for this scenario, such as

path loss, antenna gain and height or number of PRBs. The focus of the scenario is to inves-

tigate the mobility using carrier aggregation implementation. This is an important part of the

simulation, given that the carrier aggregation technique will allow connections during mobility

with the small cells operating at high frequency. This will impact the achieved data rate and

the throughput for each user.

The outcome of the simulations will be evaluated in terms of achieved data rate, throughput,

handovers and other key performance indicators (radio link failures, handover failures), as it has

been done for the previous measurements for the LTE system in low and high speed (Section 3.2

and Section 4.1). Given the outcome of the simulations performed, the influence of the chosen

frequency, base station power and trigger events parameters will be analysed. It is wanted to find

the optimum values and a good trade off between these latter parameters, so that the minimum

data rate of 10 Mbps is achieved with a small outage value.

5.1 Network Presentation

The implementation of the scenario departs from the initial measurements performed on the

highway in Aalborg which are described in Section 4.1 (p. 35). The network deployment consists

of the LTE macro cells to which the UE has been found to connect during the measurement

and an additional layer of small cells, set to fulfil the requirements of a 5G network. Therefore,

a HetNet is simulated, where the carrier frequency for the macro cells remains the same as in

the previous measurements, 1.8 GHz, and for the small cells it is set to 10 GHz (according to

50

CHAPTER 5. 5G IMPLEMENTATION

simulations performed by Nokia employees). The chosen path for the simulation remains the

same as the one from the measurements and the small cells have been manually deployed along

this path, in order to bring the network closer to the UE and thus to provide a good coverage

during the simulations. The geographical distance between every two consecutive small cells is

set to approximately 100 meters, thus 108 small cells have been deployed in order to cover the

path. The small cells are not deployed along the tunnel, which is 600 meters long. A general

overview of the deployed scenario is shown in Figure 5.1.

-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

Streets and HetNet Layout

X [m]

Y [m

]

1

2

3

4 5

6

7

8

9

10

11

12

13

14

15

16

17

18 19

20

21 22

23

Figure 5.1: HetNet Scenario with LTE and 5G

5.2 Path loss models

This subject of this section is the path loss models used in the simulation of the HetNet pre-

sented in the introduction of this chapter. For the macro layer the 3GPP path loss model has

been chosen (utilized in Section 4.1.2 (p. 42)) and for the small layer the model is the one tested

in METIS.

Assumptions have been made for choosing the antenna heights for the two models. As it has

been previously described in Section 4.1 (p. 35), the model uses the antenna height over the

rooftop. For the macro cells the base stations have uniform height of 20 meters and the path

loss is calculated according to [TR25.942, ], which was shown in Section 4.1 (p. 35). According

to the specifications, this model is used for test scenarios in suburban areas, which is why it is

chosen for the purpose of the simulations on the highway.

51


The antennas for the small cells are assumed to be omnidirectional and the height for the

corresponding base stations is chosen in accordance with the METIS model, so it is set to 10

meters from the ground level. The small cells are assumed to have an hexagonal layout and

the path loss model according to the assumption for the layout and the height of the antennas

is calculated using equation 5.1 [TR36.814, ]. This model takes into account both line-of-sight

(LOS) and non-line-of-sight (NLOS) conditions, needed for the propagation in the small cells

along the highway.

L =20 · log10(f

MHz) + 28 + 22 · log10(R)+

26 · log10(f

MHz) + 22.7 + 36.7 · log10(R)

(5.1)

Where

� f is carrier frequency (10 GHz)

� R is the separation between the UE and the base station

The first line of the equation is for the LOS condition and the second line is for the NLOS

condition. The values found for computing the path loss are used in [METIS et al., 2015, p.

15] for testing an urban micro path loss model for a frequency range between 0.8 and 60 GHz.

The results found in [METIS et al., 2015] state that the tested path loss model fits the LOS

measurements for frequencies higher than 6 GHz. However, for the NLOS case, the tested

model seems to differ a lot from the measurements. By assuming that along the highway a UE

is in LOS due to the suburban type of scenario (less reflections), the LOS map has been created

for the small layer and it can be depicted in Figure 5.2 (p. 53).

52


LOS and NLOS map for small cell layer

X [m]

Y [m

]

-2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

Figure 5.2: Line of Sight map for small layer

The red points represent the LOS conditions and the NLOS is represented in blue. The LOS

seems to fit with the network, since the line-of-sight conditions follow the deployment of the

small cells along the highway (represented in white).

In the standard [TR36.814, ] it is also specified the shadow fading standard deviation for the

urban micro scenario is LOS (σ = 3) and NLOS (σ = 4), which are utilized in the simulation.

Another parameter given by the specifications [TR36.814, ] is the antenna gain for HetNet sce-

narios, which for the small cells is 5 dBi. The antenna gain for the macro cells is as defined in

Section 4.1 (p. 35).

The simulation is done per physical resource block (PRB), notion which is described in Appendix

B. Therefore the number of PRB is set to 100 for the macro cells, as it has been found in the

measurements campaign. For the 5G context, high carrier frequency is assumed (10 GHz), with

overall bandwidth of 100 MHz. At very high carrier frequency, the coherent bandwidth is wider.

Consequently, it is assumed that the bandwidth can be partitioned over 10 MHz. Thus, there is

a total of 10 PRBs each with 10 MHz bandwidth.

An overview of the general simulation parameters is summarized in Table 5.1, taking as a

reference the paper [Barbera et al., 2012]. The tables includes the scenario setup and the user

settings.

53


Parameter Macro Cells Small Cells

Scenario setup:

Network Type 4G 5G

Carrier Frequency 1.8 MHz 10 GHz

Bandwidth 20 MHz 100 MHz

PRB Number 100 10

Base station Transmit Power 46 dBm 30 dBm

Path-Loss 124.94 + 36.8log10(R)108 + 22log10(R) for LOS

126.7 + 36.7log10(R) for NLOS

Shadowing Standard Deviation 8 dB 10 dB

Shadowing Correlation Distance 50 m 13 m

Antenna Height 20 m (from rooftop) 10 m (from ground)

Number of Cells 23 108

Traffic Model Fixed Buffer Fixed Buffer

User Settings:

Street users Number 200 200

Users Speed 120 km/h 120 km/h

Users Distribution Random Random

Background Users None None

Speed of Background Users None None

Simulation Time 400 sec 400 sec

Table 5.1: Parameters used for the mobility simulation of the HetNet with LTE and5G

5.3 Mobility with Carrier Aggregation

Since the network is a HetNet with two layers operating at different carrier frequencies, down-

link carrier aggregation is utilized in order to simulate this kind of scenario. The basic concepts

for carrier aggregation have been introduced in Section 2.3 (p. 19). The implementation of the

mobility procedures will be presented in this section. A UE entering a cell is configured by a

measurement setup with specific TTT values to this cell. The TTT is specified for each type of

cell (macro or small cells). Intra-frequency handovers between two macro cells or between two

small cells are based on the measured RSRP values. For handovers between a macro and a small

cell and vice versa, the trigger value is the RSRQ [Barbera et al., 2012]. It is wanted to analyse

the mobility in such a HetNet scenario and the impact of the offset, time-to-trigger (TTT) and

hysteresis values on the mobility. The failures in the system will be further investigated and

how can they be minimized by adjusting the values of the parameters.

The use of inter-band carrier aggregation can be observed in a heterogeneous network which can

be depicted in Figure 5.3 (p. 55).

54


Figure 5.3: Mobility with carrier aggregation in a HetNet scenario[Pedersen et al., 2013]

The base stations have different transmit power levels. Macros and small cells are assumed to

be interconnected, for example through X2 interface. Figure 5.3 shows the mobility in such a

HetNet scenario. The main handover events at the SCell layer are based on adding, changing/re-

configuring or removing an SCell.

While the UE moves under the coverage of the same macro cell (PCell), it has a constant

stable connection. The addition of an SCell is based on event A4 (neighbour cell becomes

better than threshold). For this event, the threshold is -12 dB and the TTT value is 160 ms

[Pedersen et al., 2013].

The A6 event defined in section 2.3 (p. 19) is the event that triggers the handover between two

small cells and it is configured by the network. This is equivalent with changing the current

serving SCell with another target SCell. The TTT value for the A6 event is currently set to 160

ms.

When the UE leaves the coverage of a small cell, this event is called SCell removal. In this

scenario, a new event is introduced for the removal of an SCell, which is the event A7, not defined

in the 3GPP standard. This event is triggered when the neighbour SCell becomes worse than

threshold, similar to the event A2 defined in table 2.1, from Section 2.2 (p. 8). The threshold

value for SCell removal is -17 dB and the TTT is as for the other events, 160 ms.

An overview of the parameters used to simulate the mobility with carrier aggregation is presented

in Table 5.2.

55


Parameter Macro Cells Small Cells


Trigger Event A3 -

A3 TTT 1.024/0.320 sec -

A3 Offset 2 dB -

A3 Hysteresis 2 dB -

Carrier Aggregation setup:

SCell Addition

Add Trigger Event - A4

Add TTT - 160 ms

Add Threshold - -12 dB

SCell Change

Change Trigger Event - A6

Change TTT - 160 ms

Change Offset - 1 dB

SCell Remove

Remove Trigger Event - A7

Remove TTT - 160 ms

Remove Threshold - -17 dB

Table 5.2: Parameters for mobility with carrier aggregation

After having explained how mobility works with carrier aggregation, the message exchange in

the scenario will be discussed in the next chapter.

The outcome of the simulations using the values presented in the two tables will be further

evaluated. Since the purpose of the scenario is obtaining the minimum data rate of 10 Mbps,

the throughput and data rate will be investigated, as well as how the presence of the small cells

affect it. The mobility failures and the handovers are also analysed.

5.3.1 Simulation Outcome

The scenario is created such as the UE has always a connection to a macro cell, while adding,

changing or removing the small cells as it moves in high speed. Therefore, one UE is connected

to the PCells 100% of the time, but it is connected to the SCells only 33% of the time, meaning

when adding or changing an SCell. These statistics are depicted in Figure 5.4 (p. 57) which

shows how much of the simulation time one user is connected to a SCell.

56


SCell PCell0

0.2

0.4

0.6

0.8

1

Per

cent

age

of C

onne

ctio

n pe

r U

E

Connection Statistics

Small CellsMacro Cells

Figure 5.4: Percentage of one UE connection to the cells

From the figure it can be noticed that many of the SCells which take part in the mobility are

removed or there is no SCell addition at all. The lack of SCell deployment in the tunnel area

has its contribution to this fact. Another reasoning for the little connection to the SCells is the

high speed of the UE, which doesn’t allow frequent SCell connections because their coverage is

very small compared to the one of the PCells.

In order to explain the little connection time to the SCell it is investigated which one of the

events at the 5G layer is more frequent. This is shown by the plot in Figure 5.5 (p. 58).

57


Rem Add Chg0

2

4

6

8

10

12

Eve

nts

per

UE

per

Min

ute

SCells Statistics

AdditionRemovalChange

Figure 5.5: Percentage of one UE connection to the cells

It can be noticed that compared to the addition and removal events, the UE SCell change rate

is way smaller. There are 1.56 cell changes per UE per minute compared to 11 additions and

removals per UE per minute. This means that instead of having a constant connection to the

small cells during mobility, these are added and removed frequently. The reasons for these are

the low values of the RSRP, which is the triggering quantity for the SCell change.

Throughput

The throughput that the UE experiences along the whole path is shown in Figure 5.6 (p. 59), as

it can be seen 80% of the time the throughput is below 10 Mbps. One of the reason is because

the UE does not experience a continuous connection to the small cells. One other factor that

can affect the throughput is the absence of small cell along the tunnel where the UE is only

connected to the macro cells.

58


10-2

10-1

100

101

102

103

104

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Throughput [Mbps]

CD

F

Received Throughput

Figure 5.6: Throughput outage value for the entire simulation

Mobility Failures for the Implemented Scenario

Another outcome of the simulation scenario is related the radio link failures and handover events.

The handover and failure events are only related to the PCell, while the events at the SCell layer

are only the addition, change and removal. From the simulation, it is found that there are 5.25

handovers per UE per minute and 0.56 radio link failures per UE per minute. There are three

types of RLF that can be extracted from the measurements: pure RLF (given by low SINR val-

ues), RLF during handover preparation and RLF during handover execution (handover failures).

Figure 5.7 (p. 60) shows the number of RLF per UE per minute for all the users during the

simulation time, which is 6.83 minutes. The number of pure RLF per UE per minute obtained

from low SINR values is approximately 0.5 and 0.05 during handover execution. No RLF have

been found during handover preparation.

59


Pure Prep Exec0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

RLF

per

UE

per

Min

ute

RLF Statistics

Pure RLFRLF during PreparationRLF during Execution

Figure 5.7: Number of RLF per Minute obtained from the Simulation

The TTT parameter has an impact on the overall handover time. Thus, it is wanted to find a

new configuration of the TTT value that will result in less failures. This will be the subject for

the next chapter, where the results from changing some of the system parameters will be shown.

Conclusion: The presented scenario which uses as a macro layer the real highway scenario

and small cells along the highway is not be able to reach the minimum desired data rate. The

reason for this is related to a non-continuous connection to the small cells. This means that the

parameters set in Table 5.2 (p. 56) and Table 5.1 (p. 54) need to be reexamined.

60

Chapter 6

Optimizing the scenario

This chapter presents the results obtained from changing some of the parameters from the sim-

ulation scenario described in Chapter 5. The aim for changing the parameters is to minimize

the radio link failures at the PCell layer (macro cells) and to obtain a better connection with

the small cells at the SCell layer. The latter change is done in order to improve the received

throughput at the UE end.

The findings in Chapter 5 have shown frequent addition and removing of SCells. A reasoning

for this might be that the high frequency of 10 GHz that is utilized in the scenario will give a

”spotty” coverage in terms of small cell connections. In order to fix this issue, two approaches

can be followed: either lowering the frequency or densifying the SCell network. Lowering the

frequency will result in a better coverage from the SCells and therefore the removals will not

be so frequent any more. Another approach is to keep the carrier frequency of the small cells

to 10 GHz and add more small cells in the network. For the purpose of this project it is cho-

sen to lower the carrier frequency to the minimum required for this model, meaning 6 GHz

[METIS et al., 2015].

6.1 Radio Link Failure evaluation

In this section, the RLF in the system are investigated by changing the time-to-trigger value

to shorter values than in the initial scenario. The TTT values for the A3 event are set to 480,

256 and 128 ms and the number of radio link failures is analysed for each of the values. The

reason for choosing shorter TTT is that for fast moving users the users will be experiencing

many handovers, which would lead to too early, too late or even unnecessary handovers. It is

desired to minimize the radio link failures and handover failures . In Figure 6.1 (p. 62) is shown

the impact of the TTT values on the number of RLF per UE per minute during the whole

simulation time (6.83 minutes).

61

CHAPTER 6. OPTIMIZING THE SCENARIO

0.128 0.256 0.480 1.0240

0.1

0.2

0.3

0.4

0.5

TTT Value [seconds]

RLF

per

UE

per

Min

ute

RLF Statistics

Pure RLFRLF during Execution

Figure 6.1: Handovers and RLF per UE per minute with different TTT values

As it can be noticed from the figure, the high values of TTT will result in an increased number

of radio link failures. The reasoning for these findings is that since the users move in high speed,

there is no need for high values of TTT, because the timer will not expire early enough for the

UE to trigger the handover even though a neighbouring cell might meet the triggering condition.

Given the findings from the statistics presented in Figure 6.1, it is chosen to set the TTT timer to

256 ms, since this it seems to be the optimum value for having less RLF. Then, with this setting

it will be investigated how to prolong the connection to the SCells by having more changing of

small cells than addition and removal. In order to do this, some parameters for the SCell layer

actions will be changed in terms of thresholds and TTT values.

62


6.2 Frequency evaluation

The purpose of this section is to show the effects of different frequencies on the scenario. For

this reason simulations are run for a frequency range between 6 GHz and 20 GHz. Figure 6.2

shows the trend of the outage probability to have 10 Mbps and the percentage of the connection

to the small cells for each frequency.

6 8 10 12 14 16 18 2070

75

80

85

90

Frequency [GHz]

Per

cent

age

Out

age

Pro

babi

lity

Outage Probability and SCell Connection with respect to Frequency

6 8 10 12 14 16 18 2020

25

30

35

40

Per

cent

age

Con

nect

ion

SC

ell

Figure 6.2: outage probability and SCells connection respect to frequency

The figure shows that as the frequency increases, the outage value increases as well. This means

that the probability of having the satisfaction of 10 Mbps for the UEs decreases. The reason for

this is that increasing the frequency will result in the shrinking of the propagation area in the

small cells. This can be seen in the trend represented in green in Figure 6.2, which shows that

the percentage of connection to the small cells decreases with the increase in frequency. After

the point where the two curves intersect, close to 12 GHz, it can be concluded that there is no

point in keeping to increase the frequency for this particular scenario, since the connection to

the small cells keeps dropping and as a result, the outage probability increases.

Therefore, for this specific scenario it is most convenient to choose a carrier frequency of 6 GHz,

where the outage value is minimum and the connection to the small cells in maximum.

6.3 Small cell events optimization

As it has been concluded in the previous section, the more convenient frequency for this partic-

ular scenario is 6 GHz. In this section methods to improve the scenario are investigated. So far,

from the analysis of the scenario it is clear that the the high outage probability is due in general

to a little connection to the small cells as well as a discontinuous connection to the small cells.

For these reasons, is wanted to investigate a method that can allow a continuous connection and

therefore an increase in the throughput. This leads to evaluating the parameters that trigger

the adding, removal and changing of the small cells.

63


The values of the parameters for adding, removing and changing a small cells used for the sim-

ulations so far are shown in Table 5.2(p. 56). The main idea is to decrease the value of the

threshold that adds the small cell and at the same time lower the relative TTT value in order

to trigger more frequent addition. Also the TTT and offset of the changing event are modified.

In this case the TTT value is also wanted to be smaller so that more continuous connection to

the small cells is obtained. It is wanted to make the removal difficult so that the connection to

the small cells is more constant and more changing is made possible. This is done by increasing

the TTT value as well as the threshold. In Table 6.1 the two chosen configurations are shown

for the small cell events according to the previous proposal.

Parameter SCell Conf. 1 SCell Conf. 2

Carrier Aggregation setup:

SCell Addition

Add Trigger Event A4 A4

Add TTT 120 ms 80 ms

Add Threshold -14 dB -16 dB

SCell Change

Change Trigger Event A6 A6

Change TTT 80 ms 40 ms

Change Offset 1 dB 1 dB

SCell Remove

Remove Trigger Event A7 A7

Remove TTT 250 ms 480 ms

Remove Threshold -19 dB -19 dB

Table 6.1: Different configurations for the small cell events

The threshold for removing the small cell, which is based on RSRQ measurements, is set to be

very close to the acceptable limit (-19.5 dB [Salo, 2013]).

The impact of the two configurations on the throughput with 6 GHz is shown in Figure 6.3 (p. 65).

The red and blue curve represent the throughput at 6 GHz using the event Configuration 1 and

2 respectively. The green curve instead is the throughput of the original configuration (Table

5.2 (p. 56)).

64


The figure shows that lowering the frequency brings benefits with respect to throughput, as

well as designing a better configuration for the events at the small cells. In fact, with this

combination of parameters it is possible to gain around 30-40% in throughput outage compared

to the original configuration (green curve).

10-2

10-1

100

101

102

103

104

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Throughput [Mbps]

CD

F

Throughput at different SCell configurations

SCell Conf. 1 10 MbpsSCell Conf. 2 Scell original conf.

Figure 6.3: Throughput according to the two chosen configurations

The reason for this is related to a better coverage due to the low frequency (6 GHz) together

with an optimization for continuous connection between the small cells. Figure 6.4 shows the

SCell events per UE per minute according to different configurations. Where ”Initial” represents

the original SCell values (Table 5.2 (p. 56)). ”Conf. 1” and ”Conf. 2” are the configurations

shown in Table 6.1(p. 64).

Initial Conf. 1 Conf. 20

5

10

15

20

25

Eve

nts

per

UE

per

Min

ute

SCells Statistics at 6 GHz

AdditionRemovalChange

Figure 6.4: Statistic at 6 GHz with different configurations

65


The figure shows that Configuration 2 has the best result. This means that having a really low

TTT value for adding a cell in the system is crucial, so the UE can connect to a small cell faster.

Also having a longer TTT for the removal event has a great impact. In fact, a longer TTTs

allow longer connection to the small cells and at the same time the probability that the RSRP

of the neighbour small cell is stronger increase. This leads to having more changing between the

cells and less removal and addition events.

It can be also proved that with the changing of configuration more percentage of connection to

the small cells is achieved. This is shown in Figure 6.5.

Initial Conf. 1 Conf. 20

10

20

30

40

50

60

70

80

90SCells Percentage of Connection

Per

cent

age

per

UE

Figure 6.5: Statistic for SCell connections at 6 GHz with different configurations

It can be seen in the figure that using Configuration 2, the percentage of connection to the small

cells is double compared to the initial configuration.

In order to understand how the absence of these cells along the tunnel effects the performance of

the throughput Figure 6.6 (p. 67) shows the CDF of the throughput when the UE is connected

all the time both to the SCells and to the PCells. Meaning that the samples where the UE is

connected only to the macro cells are cut away.

66


10-2

10-1

100

101

102

103

104

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Throughput [Mbps]

CD

F

Received Throughput from SCells and PCells

SCell Config 2

10 Mbps

SCell Config 2(both SCell and PCell)

Figure 6.6: Throughput when the UE is connected to PCells and SCells all the time

From this CDF plot, it can be seen that continuous connection to the SCells improves signif-

icantly the received throughput. Therefore, it is wanted to make this connection constant, by

having more SCell changing than removing and adding. This would help decrease the outage

probability with 10% for more users to get minimum 10 Mbps.

Conclusion: This section has shown that the deployed scenario in Chapter 5 can be optimized

at both network layers. The macro cell layer performance can be improved in terms of minimiz-

ing radio link failures and handover failures. This can be achieved by lowering the TTT value.

Using this improvement, the next step is optimizing the connectivity to the small cell layer. It

has been shown that it is possible to adjust the parameters of the events at the small cell layer

so that the changing event of the cells is predominant compared to the addition and removal of

cells. Therefore, having more connection to the high frequency operating cells (SCells) will re-

sult in higher user throughput. The minimum target of 10 Mbps is achieved for 60% of the users.

67

Chapter 7

Conclusion and Future Work

The focus of this project was to investigate the real performance of LTE in a high speed scenario

and compare real measurements with simulations in order to validate the used models. Also it is

wanted to verify if the actual system is be able to support the increase of the data demand, that

is expected to increase by 1000 until 2020. Furthermore it is investigated a new technology that

is able to support 10 Mbps at the user end while high speed is involved and also the mobility

performance.

From the study of the LTE system performance at low and high speed, it is possible to conclude

that LTE is resilient to radio link failures and ping-pong effect compared to the previous tech-

nology (3G). Therefore, the mobility performance of LTE is good. From the throughput point

of view, the difference between the performance at low and high speed is not so large. In fact,

on average the throughput obtained from both scenarios is around 8 Mbps. Instead the peak

value reached in low speed is on average 5 Mbps higher that the high speed.

The outcome from the simulations that reproduced the two scenario shows positive results in

terms of used models. The number of handovers in the real measurements is found to be slightly

different from the number of handovers obtained from the simulation. This is because in the

simulation the local variations of the propagated signal are not taken into account. In general

it is possible to state that the simulations have a good match with the measurements and that

the models used in this project do not alter the reality.

Another important finding is that LTE is not able to support an increase in data request in

high speed scenarios. In fact, the results from simulations show that the UE can achieve 1

Mbps 65% of the time but only the 7% of the time it can reach 10 Mbps. In order to guar-

antee minimum 10 Mbps data rate at the user end, 5G assumptions are implemented. This

implies the use of dense cells, high frequency and dual connectivity between different radio ac-

cess technologies. The macro layer (LTE) is also present in order to support the small cells (5G).

The implemented scenario uses a 4G macro layer with carrier frequency of 1.8 GHz and the 5G

small cell layer at 10 GHz. The results from simulations show that with 10 GHz and inter site

distance (ISD) of 100 metres it is not possible to reach the minimum required. In fact, only the

20 % of the users are satisfied. Another factor that influences the performance of the system is

the mobility. It is found that the UE does not have a continuous connection to the small cells

and this causes the decrease in throughput. Thus, it is necessary to have less ISD distance or

to lower the frequency. Another goal was to improve the mobility performance.

68

CHAPTER 7. CONCLUSION AND FUTURE WORK

By lowering the frequency at 6 GHz, the outage probability of 10 Mbps decreases and at the

same time the connection to the small cells becomes better. Without making any changes in

the mobility parameters the outage probability is still high (around 70%) due to a discontinuous

connection to the small cells. Thus, the mobility parameters are manipulated by lowering the

TTT value that allows (changing event) of the small cells, as well as making it difficult to leave

the cells (removal event). Also adding a cell (adding event) is configured in order to add the cells

easier. So, the combination of lower frequency together with the optimization of the mobility

performance allows to have an outage probability of 40%.

As a general conclusion it is needed to densify the network compared to the deployment pre-

sented in this project and design it such that the changing event is predominant to the removal

and adding.

From the results obtained in this work some research starting points can be developed. Fist

of all, building a new network where the inter site distance between the cells is smaller (25

- 50 metres) and evaluate the performance of this network by changing the carrier frequency.

Meaning that it is wanted to find a relationship between frequency and ISD. Furthermore a

definition of the KPI specifications for the small cells should be investigated in terms of radio

link failures, handover failures, handovers etc. Furthermore, it would be interesting to analyse

the message exchange between the UE and the eNodeBs of the macro and small cells.

69

Chapter 8

List of references

Figures and tables are produced by the project group unless otherwise specified.

List of Figures

2.1 Timeline of the mobile communications standards landscape . . . . . . . . . . . . 5

2.2 Evolution peak data rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3 Basic System Architecture Overview . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.4 Filtering at Physical and Network Layers . . . . . . . . . . . . . . . . . . . . . . 12

2.5 Handover Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.6 Handover Execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.7 Handover Completion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.8 Triggering conditions for handover in connected mode . . . . . . . . . . . . . . . 15

2.9 RACH Procedure in the Handover Process . . . . . . . . . . . . . . . . . . . . . . 16

2.10 Interruption time in the Handover Process . . . . . . . . . . . . . . . . . . . . . . 17

2.11 Timers in Handover Process and RLF Detection . . . . . . . . . . . . . . . . . . 18

2.12 Types of carrier aggregation allocation in LTE-Advanced . . . . . . . . . . . . . 19

2.13 Downlink carrier aggregation deployment scenarios . . . . . . . . . . . . . . . . . 20

2.14 Uplink carrier aggregation deployment . . . . . . . . . . . . . . . . . . . . . . . . 21

3.1 Path for the measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2 RSRP Level at 1.8 GHz Carrier Frequency . . . . . . . . . . . . . . . . . . . . . . 25

3.3 CDF Throughput at 1.8 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.4 RSRP Level at 2.6 GHz carrier frequency . . . . . . . . . . . . . . . . . . . . . . 28

3.5 Throughput CDF at 2.6 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.6 Handover position 1.8 GHz from the measurement . . . . . . . . . . . . . . . . . 32

3.7 Handover position 1.8 GHz from the simulation . . . . . . . . . . . . . . . . . . . 32

3.8 Handover position from the measurement at 2.6 GHz . . . . . . . . . . . . . . . . 33

3.9 Handover position from the simulation at 2.6 GHz . . . . . . . . . . . . . . . . . 34

4.1 Path for the measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.2 RSRP Level at 80 km/h for Path A-B . . . . . . . . . . . . . . . . . . . . . . . . 37

4.3 RSRP Level at 80 km/h for Path B-A . . . . . . . . . . . . . . . . . . . . . . . . 38

4.4 CDF Throughput at 80 km/h for both paths . . . . . . . . . . . . . . . . . . . . 39

4.5 RSRP Level at 100 km/h for Path A-B . . . . . . . . . . . . . . . . . . . . . . . 40

4.6 RSRP Level at 100 km/h for Path B-A . . . . . . . . . . . . . . . . . . . . . . . 40

4.7 CDF Throughput at 100 km/h for both paths . . . . . . . . . . . . . . . . . . . . 41

70

4.8 Macro Network at 1.8 GHz and Streets Layout in Simulations for 100km/h . . . 43

4.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.1 HetNet Scenario with LTE and 5G . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.2 Line of Sight map for small layer . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.3 Mobility with carrier aggregation in a HetNet scenario [Pedersen et al., 2013] . . 55

5.4 Percentage of one UE connection to the cells . . . . . . . . . . . . . . . . . . . . 57

5.5 Percentage of one UE connection to the cells . . . . . . . . . . . . . . . . . . . . 58

5.6 Throughput outage value for the entire simulation . . . . . . . . . . . . . . . . . 59

5.7 Number of RLF per Minute obtained from the Simulation . . . . . . . . . . . . . 60

6.1 Handovers and RLF per UE per minute with different TTT values . . . . . . . . 62

6.2 outage probability and SCells connection respect to frequency . . . . . . . . . . . 63

6.3 Throughput according to the two chosen configurations . . . . . . . . . . . . . . 65

6.4 Statistic at 6 GHz with different configurations . . . . . . . . . . . . . . . . . . . 65

6.5 Statistic for SCell connections at 6 GHz with different configurations . . . . . . . 66

6.6 Throughput when the UE is connected to PCells and SCells all the time . . . . . 67

A.1 Measurement ID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

A.2 Measurement ID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

A.3 RRC message containing the information about the measurement ID . . . . . . . 79

A.4 Message Report due to A3 event . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

A.5 Message Report due to a periodic message . . . . . . . . . . . . . . . . . . . . . . 80

B.1 (a) classical multicarrier system spectrum; (b) OFDMA system spectrum . . . . 81

B.2 Sub-carriers orthogonality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

B.3 OFDMA Transmitter and Receiver Scheme . . . . . . . . . . . . . . . . . . . . . 82

B.4 Resource structure of LTE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

B.5 SC-FDMA transmitter and receiver . . . . . . . . . . . . . . . . . . . . . . . . . . 85

C.1 Number of cells configured with two different values of A2 parameters at 1.8 GHz 87

C.2 Location of the cells configured with different A2 parameters at 1.8 GHz . . . . . 88


C.4 Location of the cells configured with different A3 parameters at 1.8GHz . . . . . 89


C.6 Location of the cells configured with different A3 parameters at 2.6GHz . . . . . 91

List of Tables

2.1 Event-Triggered Reporting Criteria for LTE and Inter-RAT Mobility . . . . . . . 14

2.2 Description of Parameters for RLF Occurrences . . . . . . . . . . . . . . . . . . . 19

3.1 Minimum and maximum values of RSRP, RSRQ and RSSI at 1.8 GHz . . . . . . 24

3.2 Different A3 event configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

71

3.3 Number of HO, RLF and PP per minute at 1.8 GHz . . . . . . . . . . . . . . . . 27

3.4 Minimum and maximum values of RSRP, RSRQ and RSSI at 2.6 GHz . . . . . . 27

3.5 Different A3 event configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.6 Simulation Parameters at 1.8 GHz and 2.6 GHz . . . . . . . . . . . . . . . . . . . 31

4.1 Minimum and maximum values of RSRP, RSRQ and RSSI obtained for 80 km/h 37

4.2 Minimum and maximum values of RSRP, RSRQ and RSSI obtained for 100 km/h 39

4.3 Cells configured with different A3 event parameters . . . . . . . . . . . . . . . . . 41

4.4 Parameters used for the high speed mobility simulation . . . . . . . . . . . . . . 44

4.5 Number of handovers per minute in drive tests and simulations . . . . . . . . . . 45

5.1 Parameters used for the mobility simulation of the HetNet with LTE and 5G . . 54

5.2 Parameters for mobility with carrier aggregation . . . . . . . . . . . . . . . . . . 56

6.1 Different configurations for the small cell events . . . . . . . . . . . . . . . . . . . 64

72

References to Books

[Holma and Toskala, 2009] Holma, H. and Toskala, A. (2009). LTE for UMTS-OFDMA and

SC-FDMA Based Radio Access. Wiley and Sons Ltd., 1. edition. ISBN: 978-0-470-99401-6.

[Holma and Toskala, 2012] Holma, H. and Toskala, A. (2012). LTE-Advanced 3GPP Solution

for IMT-Advanced. John Wiley and Sons Ltd., 1. edition. ISBN: 978-1-119-97405-5.

[METIS et al., 2015] METIS, V. N. N., (Aalto), A. K., (UOulu), A. R., HHI), L. R. F.,

DOCOMO), T. I. N., (AALTO), J. J., (Ericsson), J. M., (NSN), J. V., (EB), J. M.,

(AALTO), K. H., (UOulu), V. H., (EB), J. Y., DOCOMO), N. O. N., (DOCOMO), K. K.,

(Anite), P. K., (Anite), T. J., (Anite), A. H., HHI), R. W. F., and HHI), M. P. F. (2015).

METIS Channel Models.

[Stefania Sesia and Baker, 2011] Stefania Sesia, I. T. and Baker, M. (2011). LTE-The UMTS

Long Term Evolution from theory to practice. Wiley and Sons Inc., 2. edition. ISBN:

978-0-470-66025-6.

References to Articles and Papers

[Barbera et al., 2012] Barbera, S., Michaelsen, P. H., Saily, M., Pedersen, K., University, A.,

and Networks, N. S. (2012). Improved mobility performance in lte co-channel hetnets

through speed differentiated enhancements. IEEE Communications Magazine.

[Chavarrıa et al., 2014] Chavarrıa, L. G., Barbera, S., Polignano, M., Pedersen, K. I., Elling,

J., Sørensen, M., University, A., Networks, N., and Denmark, T. A. (2014). Validation of

mobility simulations via measurement drive tests in an operational network. IEEE 81st

Vehicular Technology Conference: VTC2015-Spring.

[Ericsson, 2015] Ericsson (2015). 5g radio access. White paper.

[Ishii et al., 2012] Ishii, H., Kishiyama, Y., Takahashi, H., and Innovations, D. (2012). A novel

architecture for lte-b. IEEE Communications Magazine.

[Jeffrey G. Andrews et al., 2014] Jeffrey G. Andrews, Fellow, I., Stefano Buzzi, Senior Member,

I., Wan Choi, Senior Member, I., Stephen V. Hanly, Member, I., Angel Lozano, Fellow, I.,

Anthony C. K. Soong, Fellow, I., and Jianzhong Charlie Zhang, Senior Member, I. (2014).

What will 5g be? IEEE Journal on Selected Areas in Communications.

[Lopez-Perez et al., 2012] Lopez-Perez, D., Guvenc, I., and Chu, X. (2012). Mobility

management challenges in 3gpp heterogeneous networks. Communications Magazine, IEEE,

50(12):70–78.

[Nokia, 2015a] Nokia, N. (2015a). Looking ahead to 5g. White paper.

[Nokia, 2015b] Nokia, N. (2015b). Lte-advanced carrier aggregation optimization. White paper.

73

CHAPTER 8. LIST OF REFERENCES

[Pedersen et al., 2013] Pedersen, K. I., Michaelsen, P. H., Rosa, C., Barbera, S., University, A.,

and Networks, N. S. (2013). Mobility enhancements for lte-advanced multilayer networks

with inter-site carrier aggregation. IEEE Communications Magazine.

[Salo, 2013] Salo, J. (2013). Mobility parameter planning for 3gpp lte: Basic concepts and

intra-layer mobility.

[Song et al., 2014] Song, H., Fang, X., and Yan, L. (2014). Handover scheme for 5g c/u plane

split. heterogeneous network in high-speed railway. IEEE Transactions on Vehicular

Technology.

[TR25.913, 2008] TR25.913, G. (2008). Technical specification group radio access network;

requirements for further advancements for e-utra (lte-advanced). 3rd Generation

Partnership Project (3GPP).

[TR25.942, ] TR25.942, G. Technical specification group (tsg) ran wg4; rf system scenarios.

(25.942).

[TR36.211, 2007] TR36.211 (2007). Evolved universal terrestrial radio access (e-utra); physical

channels and modulation. 3rd Generation Partnership Project (3GPP).

[TR36.214, 2011] TR36.214 (2011). Evolved universal terrestrial radio access (e-utra); physical

layer; measurements. 3rd Generation Partnership Project (3GPP).

[TR36.331, 2012] TR36.331, G. (2012). Evolved universal terrestrial radio access (e-utra);

radio resource control (rrc); protocol specification. 3rd Generation Partnership Project

(3GPP).

[TR36.814, ] TR36.814, G. Technical specification group radio access network; evolved

universal terrestrial radio access (e-utra); further advancements for e-utra physical layer

aspects. (36.814).

[TR36.839, 2012] TR36.839, G. (2012). Technical specification group radio access

network;vevolved universal terrestrial radio access (e-utra); mobility enhancements in

heterogeneous networks. 3rd Generation Partnership Project (3GPP).

References to Notes and Others

[Anritsu, URL] Anritsu (URL). Understanding lte-advanced carrier aggregation.

http://africa.comworldseries.com/files/93031-Understanding-Carrier-Aggregation-web.pdf.

Issue 2, 09/2013.

[Kathrein, 2009] Kathrein (2009). 65° panel antenna.

http://www.kathrein-scala.com/catalog/80010644.pdf. Model 800 10644.

[TechBarnWireless, URL] TechBarnWireless (URL). Lte measurement.

http://techbarnwireless.blogspot.fi/2014/01/lte-meas.html. Date of application:

12. April, 2014.

74

http://africa.comworldseries.com/files/93031-Understanding-Carrier-Aggregation-web.pdf

http://www.kathrein-scala.com/catalog/80010644.pdf

http://techbarnwireless.blogspot.fi/2014/01/lte-meas.html

Chapter 9

Annex Index

The annex is to be found on the attached CD. This includes the articles and material used in

the project.

ALL ANNEXES ARE ON THE ACCOMPANYING CD

A list of folders and their contents are listed below:

List of articles and notes

� 3rd Generation Partnership Project; Technical Specification Group (TSG) RAN WG4; RF

System Scenarios

� 3rd Generation Partnership Project; Technical Specification Group Radio Access Network;

Evolved Universal Terrestrial Radio Access (E-UTRA); Further advancements for E-UTRA

physical layer aspects (Release 9)


Evolved Universal Terrestrial Radio Access (E-UTRA); Mobility enhancements in hetero-

geneous networks (Release 11)

� LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and mod-

ulation (3GPP TS 36.211 version 10.0.0 Release 10)

� LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer; Measurements

(3GPP TS 36.214 version 10.1.0 Release 10)


Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC);

Protocol specification (Release 9)

� A Novel Architecture for LTE-B. C-plane/U-plane Split and Phantom Cell Concept -

Hiroyuki Ishii

� Handover Scheme for 5G C/U Plane Split. Heterogeneous Network in High-Speed Railway

- Hao Song, Xuming Fang and Li Yan

� Improved Mobility Performance in LTE Co-Channel HetNets Through Speed Differentiated

Enhancements - Simone Barbera, Per Henrik Michaelsen, Mikko Saily*, Klaus Pedersen

� LTE-Advanced Carrier Aggregation Optimization - Nokia Networks

� METIS Channel Models - Deliverable D1.4

� Mobility Enhancements for LTE-Advanced Multilayer Networks with Inter-Site Carrier

Aggregation - Klaus I. Pedersen, Per Henrik Michaelsen, and Claudio Rosa, Nokia Siemens

75

CHAPTER 9. ANNEX INDEX

Networks Simone Barbera

� Mobility Management Challenges in 3GPP Heterogeneous Networks - David Lopez-Perez,Ismail

Guvenc, Xiaoli Chu

� Mobility Parameter Planning for 3GPP LTE: Basic Concepts and Intra-Layer Mobility -

Jari Salo

� Universal Mobile Telecommunications System (UMTS); LTE; Requirements for Evolved

UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN) (3GPP TR 25.913 version 8.0.0

Release 8)

� Understanding LTE-Advanced Carrier Aggregation - Anritsu

� Validation of Mobility Simulations via Measurement Drive Tests in an Operational Network

- Gimenez, Lucas Chavarria; Barbera, Simone; Polignano, Michele; Pedersen, Klaus I.;

Elling, Jan; Sørensen, Mads

� What Will 5G Be? - Jeffrey G. Andrews

List of other material

� Project Proposal - Design of optimized mobility capabilities in future 5G systems

76

Appendix A

Phone messages

In this part of the report is explained the message exchange between the UE and the eNodeB

which are involved in the handover procedure. This helps to find the values of the events A2, A3

and A5. First of all, it will be explained how the UE reports the measurements. Then, based

on this, the messages the contain the handover command will be presented.

A.1 Measurement report

In this section is presented how the phone (UE) reports the measurements to the eNodeB. In

general, the UE performs different types of measurements[TR36.331, 2012, p. 73]:

� Intra-frequency measurements

� Inter-frequency measurements

� Inter-RAT measurements

For the measurements described in Section 4.1 (p. 35) and Chapter 3, the type measurements

that is taken in account is intra-frequency. This is because the phone is forced to work in only

one frequency during the measurements. By analysing the messages got from the measurements

taken in Aalborg, it is found that the measurement report can be: periodic or event triggered,

for example when the event A3 occurs (see the definition in the Table 2.1). In order to recognize

the related messages to the type of measurement, the UE uses different measurement ID.

Figure A.1 is utilized to map the measurement ID to the type of measurement performed by the

UE.

Figure A.1: Measurement ID [TechBarnWireless, URL]

As figure A.1 shows, the measID is the combination of two elements:

� measuObjectID: identifies which carrier frequency is measured and therefore the network

77

APPENDIX A. PHONE MESSAGES

� reportConfigID: this identifies the cause of the measurements (e.g events, periodical mea-

surement)

For example, the measID = 1 refers to a measurement in a LTE network with a code carrier

frequency equal to 1212 due to the A3 event. The measID = 3 refers to measurements in a 3G

network with a code carrier frequency equal to 5656 due to the event B1 (inter RAT neighbour

becomes better than a threshold).

So, taking as a reference the table in Figure A.1 (p. 77), it is possible to extract the informa-

tion stored in the measurement used for this project. This information has been found in the

RRCConnectionReconfiguration messages.

measIDmeasObjectID

reportConfigID

1

2

4

9

1

121413

1

measObjectmeasObjectID carrierFreqmeasObjectEUTRA 18501

ReportConfID reportConfig Event

1

2

3

4

ReportConfigEUTRA

ReportConfigEUTRA

ReportConfigEUTRA

ReportConfigEUTRA

A3

A5

Periodic

A2

Figure A.2: Measurement ID

From Figure A.2, it can be noticed that the measObjectID is always equal to 1. This is because

the measurements related to this thesis are taken only for LTE network and only for one frequency

(2.6 GHz or 1.8 GHz). On the other hand, the ReportConfigID has four values:

� ReportConfigID = 1: refers to the A3 event


� ReportConfigID = 3: refers to the a periodical measurement


So it is possible to summarize the measurement report ID as follows:

� measID = 1: refers to a LTE measurement caused by the A3 event


� measID = 4: refers to a LTE nett due to periodical measurement


As a confirmation of the discussion above, in Figure A.3 (p. 79) is depicted an RRC message

that contains the information about mapping the ID measurement.

In the measurements that have been performed for this project two measurement reports were

found: triggered by the A3 event and periodic. In the figures below these two cases are shown,

where the respective measurement ID is highlighted.

78


Figure A.3: RRC message containing the information about the measurement ID

Each time that the phone changes cell it receives from the base station an RRC connection

reconfiguration message. This contains all the information about the connection, including the

measurement configuration. The next section will expand this discussion.

79


Figure A.4: Message Re-port due to A3 event

Figure A.5: Message Report due to aperiodic message

A.2 RRC Connection Reconfiguration

According to the standard [TR36.331, 2012, p. ], RRC CONNECTION RECONFIGURATION

is the command that modifies RRC connection. The purpose of RRC CONNECTION RECON-

FIGURATION is to perform handover, to setup/modify/release measurement, dedicate NAS

information and to establish/modify/release Radio resource configuration.

In the RRC Connection reconfiguration message the measurement configuration parameter is

used to configure the measurements for the UE and the measurement report is used for report-

ing. The reporting configuration contains the reporting criteria and configuration for a UE to

trigger a measurement report. The measurement report can be triggered by and event or it can

be a periodic measurement [Holma and Toskala, 2009]. These concepts have been presented in

Appendix A.1.

Based on the measurement result received from the UE, the eNodeB triggers the handover. For

the measurements described in Section 4.1 (p. 35) and Chapter 3 where intra-LTE handovers

are performed, the RRC connection reconfiguration message is used as a handover command.

80

Appendix B

Overview of OFDMA and SC-FDMA

The following section covers the LTE multi carrier technology that are used in LTE. In downlink

the multi access is based on Orthogonal Frequency Division Multiple Access (OFDMA), in uplink

the multi access is based on Single Carrier Frequency Division Multi Access (SC-FDMA).

B.1 OFDMA

Orthogonal Frequency Division Multiple Access (OFDMA) is a special case of multicarrier sys-

tem.

In general multicarrier divides the channels in a certain number of sub-channel separated by

guard-band in order to avoid the selectivity in frequency. The advantage of OFDMA is that

the carrier frequencies are orthogonal among each other and this allows to save bandwidth and

therefore makes OFDMA highly spectral efficient [Stefania Sesia and Baker, 2011, p. 123]. The

different can be seen in figure B.1.

Figure B.1: (a) classical multicarrier system spectrum; (b) OFDMA system spectrum[Stefania Sesia and Baker, 2011, p. 124]

The reasons why the OFDMA is used in LTE are mentioned below:

� good performance in frequency selective fading channels;

� link adaptation and frequency domain scheduling;

� good spectral properties

� increased user data rates

As mentioned above, the carrier frequencies in OFDMA are orthogonal, a more exact repre-

sentation can be seen in figure B.2 (p. 82), where the center frequencies of the each sub-carrier

is chosen in order that the neighboring sub-carriers have zero value at the sampling instant of

81

APPENDIX B. OVERVIEW OF OFDMA AND SC-FDMA

the desired sub-carriers. In LTE, the distance in frequency among each sub-carrier is 15kHZ

[Holma and Toskala, 2009, p. 69].

Figure B.2: Sub-carriers orthogonality [Holma and Toskala, 2009, p. 69]

In order to understand how the OFDMA works is essential to analyze how the signal is trans-

mitted. The OFDMA signal is based on the usage of FFT (Fast Fourier Transform ) and IFFT

(Inverse Fast Fourier Transform). A schematic representation is given in figure B.3, where the

data symbols is first serial-to-parallel converted and further are send in the IFFT block. To each

Figure B.3: OFDMA Transmitter and Receiver Scheme [Holma and Toskala, 2009,p. 71]

input of the IFFT will be assign a particular sub-carrier. After the IFFT block a cyclic prefix is

added, in order to avoid inter-symbol interference (ISI) caused by multipath propagation. The

cyclic prefix is a copy of the last part of the symbol which it is added at the beginning of the

82


symbol.

Thus, based on what it has been explain before, the transmitted signal in time domain (due to

the IFFT) is the sum of all these subcarriers, and the amplitude of this signal can be very high

if they are summing up on phase.

At the receiver the reverse operations are executed to demodulate the OFDM signal. The cyclic

prefix is removed from the signal and after the serial to parallel conversion, the FFT is applied

in order to recreate the original signal.

Another important aspect for the OFDMA system to take into account is how the transmission

resource are allocated to the users. This will be clarify in the next paragraph.

Resource Structure

In downlink, the LTE standard defines a resource allocation structure in time and frequency

domains thanks to the usage of OFDMA. This paragraph will show how the symbols are trans-

mitted in LTE.

In the time domain the LTE transmissions are organized into frames of 10 ms length, where

each frame is catheterized of 10 subframes with a duration of 1 ms. These subframes are made

of two slots of 0.5 ms each. Each slot is composed of seven or six OFDM symbols depending

upon whether a short or long cyclic prefix is used.

In the frequency domain, 12 sub-carriers are grouped together (which occupy a bandwidth of

180kHz since the 12 sub-carriers are spaced at 15kHz) over a time slot of 0.5 ms. This resource

representation is called Resource Block (RB). It is possible also identify the smallest unit of re-

source with the name of Resource Element (RE), which consists of one subcarrier for a duration

of one OFDM symbol.

So, one RB consist of 84 RE when a normal cyclic prefix length is used (7 symbols · 12 subcar-

riers), otherwise is 72 RE for the extended cyclic prefix.

Figure B.4 (p. 84) shows the resource structure.

83


Figure B.4: Resource structure of LTE [Stefania Sesia and Baker, 2011, p. 146]

In summary, the minimum resource that an user can get is one RB: one single user can get

minimum 0.5 ms in time domain and 180 kHz in frequency domain.

B.2 SC-FDMA

SC-FDMA is the access used for the uplink which uses some principles of OFDMA in order to

achieve a good spectral efficiency. As it has describe for the OFDMA, in this subsection it will

be explain the principles of SC-FDMA. In the figure B.5 (p. 85) it is depicted the transmitter

and receiver schema for SC-FDMA system.

84


Figure B.5: SC-FDMA transmitter and receiver [Holma and Toskala, 2009, p. 146]

The fist step is to perform a DFT operation of the data symbols in order to produce a frequency

domain representation. After that, it maps each of the DFT output in the N orthogonal subcar-

riers. As in OFDMA, an inverse Fourier Transform (IFFT) convert the subcarriers amplitude

in to time domain signal. The last step for the transmission is insert a CP in order to avoid ISI.

therefore, each symbol is sent one at a time in the air.

The inverse operation is required at the receiver to demodulate the SC-OFDMA signal.

Resource Structure

The resource block in uplink is similar to the downlink. The frame consist in 20 slot each of

them of 0.5 ms and one subframe is formed by 2 slot. The sub-carriers are grouped into sets

of 12 sub-carriers, spaced of 15 kHz among each other, corresponding to the uplink resource

blocks. The 12 sub-carriers in one slot correspond to one uplink resource block, as the same in

the downlink part.

85

Appendix C

Statistics of the configured events for Aalbog

city center scenario

In this part of the report some statistic from the measurement done in Aalborg city center are

presented. The absence of link failures in the system during measurements needs to be justified.

For this reason, the subject of this section is the analysis of the time-to-trigger, hysteresis and

offset parameters used in the mobility procedure. This is done statistically, meaning that it is

investigated how the values of the parameters change from cell to cell. Each cell to which the

UE connects is configured with different parameters.

After analysing the messages, it has been noted that at the beginning of the measurements the

UE receives an RRC Connection Reconfiguration message that includes all the values of the con-

figured events. Three events are found to be configured during the measurements: A2, A3 and

A5. Each cell has a different configuration for these events depending on the carrier frequency

in which the cell operates (either 1.8 or 2.6 GHz).

Each time the UE makes a handover from a serving to a target cell, it will receive an RRC

Connection reconfiguration message with the configuration of the target. From the message

analysis it is found that all cells operating in both 1.8 and 2.6 carrier frequencies are configured

with the same values of the parameters for the A5 event. The values for A5 are as follows:

� TTT = 640 ms

� Hysteresis =2 dB

� RSRP Threshold 1 = -106dBm

� RSRP Threshold 2 =-105dBm

As for the A2 and A3 events, their parameters have different values depending on the cell con-

figuration. Therefore, the subject for this appendix is to investigate the different configurations

of the cells extracted from the measurements. The focus is on the values of the TTT, since this

parameter has an impact in the handover event and it can be used to explain the absence of

radio link failures.

C.1 Statistic for Cells Operating in 1.8 Ghz

� Event A2

At 1.8 GHz carrier frequency, there are two possible configurations for the parameters of

the event A2. The two configurations of A2 are shown in Figure C.1 (p. 87), along with

86

APPENDIX C. STATISTICS OF THE CONFIGURED EVENTS FOR AALBOG CITYCENTER SCENARIO

the respective values for each configuration. The figure also shows how many cells are

using each of the A2 configurations.

Event A2 Event A20

2

4

6

8

10

12

14

16

18

20

Event

Num

ber

of c

ells

Numer of cells according to different values of A2 (1.8 GHz)

TTT640 ms

HYST1 dB

RSRPTHRES

-118 dBm

RSRPTHRES

-130dBm

TTT640 ms

HYST1 dB

Figure C.1: Number of cells configured with two different values of A2 parameters at1.8 GHz

It can be noted that for the 17 cells operating in 1.8GHz, the TTT (640 ms) and hysteresis

(1 dB) values remain the same for all cells. The RSRP threshold is the only one that

changes: -118/-130 dBm. In order to visualize how each cell is configured, the plot in

Figure C.2 (p. 88) is used. The information about the cells to which the UE connected

during the measurements has been collected and they are represented with different colors

(green and brown), according to the different A2 configurations. The rest of the LTE 1.8

GHz cells are represented in white.

87


-1500 -1000 -500 0 500 1000 1500 2000 2500

-2500

-2000

-1500

-1000

-500

0

500

1000

Cells with the same configuration

X [m]

Y [m

]

−118dBm

−130dBm

RSRP Threshold

Figure C.2: Location of the cells configured with different A2 parameters at 1.8 GHz

The cells represented with green are the cells for which the RSRP threshold is -118 dBm

and the cells represented with brown have the RSRP threshold equal to -130 dBm. The

chosen path is also included and it is shown with the black line.

� Event A3

For the event A3 at 1.8 GHz the configuration of the parameters is shown in Figure C.3 (p. 89).

There are also two possible configurations for this event, given by the change in the TTT

values.

88


Event A3 Event A30

2

4

6

8

10

12

14

16

18

20

Event

Num

ber

of c

ells


TTT1024ms

HYST2 dB

OFFSET2 dB

HYST2 dB

OFFSET2 dB

TTT1280ms


For the 17 cells the hysteresis and offset values are the same: 2 dB. The value of the TTT

is changing for one cell (1280ms) and remains the same for the others (1024ms). The

map of all the 4G cells in 1.8 GHz is used to localize the different cell configurations for

the event A3 and it can be depicted in Figure C.4, together with the path chosen for the

measurements.

-1500 -1000 -500 0 500 1000 1500 2000 2500-2500

-2000

-1500

-1000

-500

0

500

Value per cell

X [m]

Y [m

]

1028ms

1024msTTT

Figure C.4: Location of the cells configured with different A3 parameters at 1.8GHz

In this figure, the cells represented with green are the ones with the TTT value of 1024 ms

89


and the brown cell has the TTT equal to 1028 ms. The one cell with different configuration

is localized beyond the fiord.

After having performed the statistic for the configurations of the cells and knowing that the

handover is being triggered by the A3 event (Section A (p. 77)), it can be concluded that the

lack of radio link failures in the measurements is due to the high values of the TTT (more than

1 second). This allows more time for the handover to be triggered. Having a larger TTT also

ensures that the level of the RSRP of the neighbour cell is strong enough to avoid a handover

failure. Further on, the same procedure is used to explain the lack of failures at 2.6 GHz carrier

frequency.

C.2 Statistic for Cells Operating in 2.6 Ghz

� Event A2

During the measurements, five cells to which the UE connects have been found while

setting the carrier frequency at 2.6 GHz. All the cells have the same configuration for the

event A2:

– TTT = 640ms

– Hysteresis = 1dB

– RSRP Threshold = -118dBm

� Event A3

In the case of the event A3, two different configurations have been reported at 2.6 GHz.

They can be depicted in figure C.5.

Event A3 Event A30

1

2

3

4

5

6

Event

Num

ber

of c

ells


TTT640ms

TTT640ms

HYST3 dB

OFFSET3dB

OFFSET2 dB

HYST2 dB


90


The first configuration can be found in three of the cells and has the TTT value equal to

1024 ms and the hysteresis and offset 2 dB. The other two cells have another configuration,

with 320 ms TTT value and hysteresis and offset of 3 dB. They can also be localized on

the 4G map containing the cells which operate at 2.6 GHz, in figure C.6.

-1000 -500 0 500

-1400

-1200

-1000

-800

-600

-400

-200

0

200

400

Value per cell

X [m]

Y [m

]

Config 1

Config 2

Configurations

Figure C.6: Location of the cells configured with different A3 parameters at 2.6GHz

It can be noted from the figure that a big part of the path used for measurements is covered

by the cells with the high value of TTT(1024ms), represented in green. For this reason it

can be explained the absence of radio link failures also when operating at 2.6 GHz carrier

frequency.

91

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Design of optimized mobility capabilities in future 5G systems